The +5 TI attempts benefit unlocks at lvl 5 friendship. Nothing more can be gained from that feature.

i asked because of why we can improve it to 7 or even more lvl. why? why?)

I'm not sure about the reason either.

Not much, except people in your friend list are sorted by friendship level, so easier to start conversations with high level friends, but that's it.

in merges server you need to make it into level 6 higher friendship into other server player in your group so you can apply support with them when you doing your convoy for 2x ..

You also get small stat bonuses and trophies in the Reputation Hall interface for having a certain number of friends at higher levels:

ooh, that explains it.