I use it mainly because you can select specific parts of a model instead of having to look through the model for the specific part.
I personally donāt use it however I believe it is for the wide range of tools available, unlike standard studio you can be slightly more specific about your move / rotate dimensions.
Another benefit for F3X is that if you have a model / multiple parts selected then you can rotate around a certain point.
Itās easy to access different tools without looking all over your screen, very simple and you can also export and import buildings easier than saving to Roblox! I find F3X to be the best building tool other than Studio Build Suite being the best for rotating parts around a certain point.
No idea, Iāve always used the roblox studio tools in general, and i think its better to learn the ropes around studio more rather than relying on some F3X plugin
Many current builders use F3X because itās the tool they started out with. Iām sure you all remember but Roblox had personal servers a white ago which were eventually removed. These personal servers often held F3X contests/building contests that pretty much always used F3X, I can confidentially say that quite a few current builders as well as I started out building in personal servers.
I personally donāt like using F3X I find it quite annoying. I have a friend who likes building in F3X and itās apparently easier for detail and building in general, Iām guessing itās because youāre seeing in in your own perspective in the game but can be useful at times however I personally donāt like it. 
F3X is totally based on preference. Personally I use the built-in tools and there might be an update improving it in the future which could be a huge plus. Overall I believe others prefer itās easier for accessibility and more functional to use which is why it is such a popular program.
Hereās what I suggest, try both tools out for a while, see which tool feels nicer and observe the build you created.
Honestly, f3x is very good in terms of making a build because you just need to press a button(E.g. Z or X) to rotate or move the brick. Itās easy and simple. I will definitely recommend it. However, each person has their own preference, so if youāre more comfortable using something else, then go ahead!
If you want to use building plugins Iād very much recommend something else than F3X as its UI is overbearing and can be really messy. Iād suggest something else like qCmdUtl.
Itās really helpful for beginners, but Iāve adapted to studio from previously using F3X.