Photo etch are thin detail sets made of brass (usually). They are flat and come on a rectangular piece referred to as a "fret". They replicate inaccurate or missing detail on the kits. They have some realistic grills and mesh for engine intakes. They add a scale thickness to some pieces like armor plate or fenders.

Well, I think almost everything's been covered. It's a matter of choice, much like using a metal barrel, which I personally don't find a need for unless it comes in the kit.
One thing that has not yet been mentioned, but might help you should you decide to go on using PE, is to anneal it first.
Also, for pieces which need to be bent, such as US pioneer tool rack, the corners don't always meet quite right, and you end up having to fill the corners with CA. Sometimes I like to just use the PE as a master, and cut pieces of .005 styrene to replace the PE. In some cases, you get better joints, and there's no bending.

Personally I agree with Alan here. I've never really felt the need to buy an AM set. I've seen "full-on" models @ shows, & they do look spectacular, but as others have said here, when it boils down to it you're building for yourself, so you do what you want.
I think that if you want to/are capable of building to show standard, you are going to have to use it nowadays to have any chance of winning something (I don't!!).
What you do have to watch with some of the AM people is that they get carried away & start doing parts that actually don't lend themselves to PE, eg I remember reading a review where some tilt frames were done in PE - the reviewer pointed out that on the real vehicle these were round sectioned - so watch out! In the old days we used old toothpaste or paint tubes chopped up, the old ones were soft metal & were great for chopping up to make rifle slings, tool straps etc & much cheaper!

I find that my fingers are too fat for the really tiny PE pieces. I use a magnifying lens with a light to help and I still can't seem to get some of those %&*^*^ pieces to fit together!!!!
Using PE isn't always neccessary for a good looking model. OOTB building sometimes looks better.
The secret is to work slowly and if you get frustrated, just take a break and relax, walk away and come back in a half hour or so to try again.

I use PE. I like Aber over all the rest. Eduard to me is inferior and while Royal Model is good stuff it is astronomically expensive. Lion Roar (are they even still around?), Part and a couple of others are out there as well. If you are just learning I would not worry about using it too much PE right now, although the more you use (whether successfully or not... at first), the better you ought to get at handling it.

I like using photo-etch because it helps me dig in to the actual design and make up of the real vehicles, as plastic kit manufactures often take short cuts due to mold limitations (and budget?). The smaller sub-assemblies of PE detailing exposes you to more of the actual components that make (made?) up the real vehicle than say, the plastic kit's offering, which usually combines and simplifies several aspects of a component into one or two pieces. Because it's all in one lump, one may not even notice some of the details because attention may not be very focused. Looking at the instruction sheet you merely glue part 49 into part 50 without really seeing what makes up those two parts. Within this process, some of the more intrinsic and intersting details that make up the real vehicle will never become apparent to you - because the process is too simple and your need to really "LOOK" at the parts that make up the AFV is not really twigged into motion.
When using PE you need to build things up in smaller bits and sub assemblies and while doing so you may need to dig into reference, and really look closely to clear up confusion on how one part goes to another. Because of the deeper digging into reference info, one really "learns" about the vehicle and it's components and construction.
I am using a lot of PE (three sets!!) on a build I'm doing over at planetArmor right now RSO PAK40
and it's really helping make the Italeri RSO PAK 40 look better and makes it a far more accurate model. It also is really helping me "get to know" this vehicle, way more than if I had just built it straught out of the box.

I would, if I were you and a beginner just honing my skills, make it easy on myself and just build out of the box for 10 kits or so and with that you can gain a good base of techniques that after awhile perhaps more challenging techniques can be added as time goes on. Take a look here at Bruce Williams's work- all OTTB I believe.
Bruce Williams
They all look great. Maybe for now concentrate on getting the kits together and painted? Work on weathering techniques.

If you are just learning I would not worry about using it too much fancy PE or resin aftermarket frills although the more you use, the better you ought to get at handling it.
PE is also expensive.
Also it may be too much (too difficult) for you right now and the frustration of trying to use it unsuccessfully now may taint your opinion on it. This may cause you to avoid it in the future, causing you perhaps to miss out on a useful augmentation to accuratizing your military miniature kit projects. Work on just getting the plastic kits together and painted/weathered. Good luck!

Yes, they now do small sets of PE for the new Tamiya 1/48 scale armor and even include an aluminum barrel in the set. I have the ones for the Tiger I, Sherman and StuG III.