There are already some answers I really like, still maybe my story and view from a pretty fresh not-even-user of Gentoo helps a bit.
aleksandr-mozg wrote:Somehow, somehow you can not see any difference in comparison with the same arch linux.
If "you" means me, you're wrong

I do see an important difference: afair arch has binary packages. Hence I as user am kind of bound on what is in the repositories (I know that I can compile there, too, but at least at Ubuntu it is kind of - well, not really fitting the workflow)
aleksandr-mozg wrote:Well, supposedly you installed either gento or arch from the binaries, or even assembled your own (LFS).
Let's say I am installing (with the help of) Gentoo right now, in pieces, because of different reasons
aleksandr-mozg wrote:But who forbids you to delete everything unnecessary in these systems or configure everything that is needed or compile programs from source code?
No one, but why should I?! I mean, where is the point of installing something packed in a distribution first only to uninstall it later on because I know that I'll never use it? And why should I compile on a binary-package-based distribution with all it's hassles on administering the system later on (and there's always something with the from source compiled programs...) if I can have it here "for free"?
aleksandr-mozg wrote:You can compile in any Linux, what is the difference guys, explain who is alive?
The difference for me is more freedom and independence.
Now, my story:
When I had my first contact with a PC, Linux didn't even exist. We had MS-DOS (or DR-DOS), later on Windows 3.11. I think it was with Win95 or so, maybe the Millennium edition, that I came across SuSE first time. There was something on my PC that didn't work in Win, don't remember, what it was. Anyway, I found SuSE, not working out of the box directly, but I found
everything I needed in the internet. And I was like "wow, all written clearly and I even could create my own eye candy on the desktop like windows etc. if I just had time!". I also learned a lot about my PC.
So, I happily worked, with SuSE, with Caldera, with RedHat, whichever distribution liked to work in my at that time current PC, tipped once or twice my toes into the cold waters of Gentoo but at that time documentation for me didn't work. Later switched to Ubuntu, all was fine.
At one point Ubuntu switched something and I didn't find all the configuration files any more, but well, it worked, so what. Must have been a switch from OpenRC to systemd, but this I found out only lately on my second serious try this time with Gentoo

At one point the maintainers of Ubuntu also decided to switch to PulseAudio, for me a constant pain in the b.... Ubuntu is a nice distribution with nice people who help you along; they all do their very best to make it simple
for the user (Arch's approach for kiss from my point of view strives more to keep the system simple and slim, but that sometimes makes it difficult for the user). But this ease of use has it's backfire.
Ubuntu has over time become more and more similar to Windows in some ways. Configs often aren't clear text any more, Pulse - well, let's not talk about that one (I like to record music sometimes, not high end but Pulse still doesn't help at all for that) and every time I'd like to have a new version of a program I either need to add the repository (which is not recommended) or have to wait until it's packed into the official repositories - often enough that doesn't happen for the current LTS and I'd need to install / upgrade to the actual version, no matter if it's long term or one of the other releases.
Gentoo is made for compiling packages and still getting the dependencies sorted out, so what I hope is that I can keep my system up-to-date without having to install a new version of the complete OS every other time when I want to have the new version of Musescore, for example. And it comes with almost nothing pre-installed, compared to a distribution. So I can choose what I want. Only one editor for x, no darktable when I prefer digicam anyway, no "LMMS" (don't even know what this program does but it came with my Ubuntu) and a ton of other programs I really never ever klick on in the menu. Oh, besides, only about three fonts or so (this might change with LibreOffice, though - yes, I use that one indeed a lot). Additional benefit: I learn a lot about my PC (and kernel options

) again.
LFS - well, learning curve is still to steep for this one. So, Gentoo seems to be the one for me right now. The time I save because I don't have to uninstall a ton of useless (for me) packages or search for other stuff I can easy put into compiling.