Whats the benefit of a 100% save?

Because 100%ing games is fun.

its fun to have a 100% save thats not yours?

Depends on the game. For games like fighters with large rosters, for example, that you just want to pick up and play with a friend, the benefits are pretty immense. I do agree that it defeats the purpose of playing in a lot of different types of games, but I was able to come up with an example to the contrary without even really thinking about it.

Kind of what i said with "have all items early" but those games are only a small percentage of the actual 100% saves distributed. The latest example would be LA Noire, here on this forum and i just cant wrap my head around it

I only download them if, for whatever reason, I'm unable to transfer my own 100% save to whatever new destination I want. I had to do this recently for a couple of N64 saves after I got an Everdrive, since I don't have any way of dumping my individual cart saves. I like to return to games that I've 100% completed and wander around the various unlocked areas, and a lot of games lock most of their features behind progression, so it's very useful to have the ability to access that stuff right away.

Fighters, racers, games that you've already 100%-ed but another save has limited-time items (either legit or hacked in), Super Smash Bros., etc.

Or for some speedrunners, a save that allows for easy and consistent NG+ running; for example, Ratchet & Clank trilogy. Not every 'runner is interested in NG+, but it's beneficial for those who do.
Not to mention edited save files for Super Mario Sunshine that remove the lengthy and unskippable cutscenes in the beginning.

As you head towards it is very much game dependent, and from a person's given personal perspective some things may be undesirable that others would like (Far Cry 3 once all the map is "safe" is quite boring compared to the game where only parts are safe and others you have to be wary/fast/violent, however I can see others just wanting to cruise around and there have been open world driving games in the past where I preferred it).

Usual things are
New game plus unlocked.
Potentially all the map to explore rather than starting area, possibly with high level/max level characters
If the game does a poor job of showcasing end game mechanics early on (and it is a tricky aspect of game design that very few devs appreciate) then you also might have those to play with, even more so if you played a prequel/sequel and want that feeling back.
Any boring collectathons done if doing the true 100%. For me number of collectathons I have enjoyed in a game... probably armour pieces in Bomberman 64 (though they were heavily dependent on creative use of mechanics and more of a puzzle than anything else) and Skulltulas in Zelda 64 can be OK I guess, Far Cry 3 animal hunting was sort of maybe OK but it tended to be gated off for later missions before it got really good (though for game balance reasons -- hunting gains you extra ammo capacity), was playing games long before and have done ever since and those are the only real examples I have. I also love platformers, open world, RPGs, puzzle games, abusing mechanics to do odd things and even when not abusing mechanics my "refusal of the call" extends to absurd lengths -- I played skyrim for 80 plus hours and never unlocked a shout, was far from the first game like that where I only tripped over and started the story way late into things with Might and Magic 7 probably being my favourite of those for that sort of thing.
Pointless unlock sequences also done before things start. Fighting games come to mind here, even more so if you want to be learning together with someone and then my 30 hours of unlock practice render it pointless, with racing ones much the same (though that for me usually falls under fun end game content).

I would say more games than not have reasons to download a completed save. Especially if you've already completed the game before and just want to recover what you had. It's probably pretty rare for someone to download a 100% save file for a story-driven game they've never played unless there's something specific it unlocks. Like if beating the game unlocked a higher difficulty you wanted to start from if you're weird like me and enjoy playing games on their highest difficulty.

Even a Tales game for example if you JUST wanted the story you could get a completed save file with a ton of grade and just blow through with max level characters and the best equips.