Not benefit, but benefits. It's a rule of thumb. Off the top of my head, some of them are: rack up xp, united front bonus from autocracy, diplomacy bonuses ("the enemy of my enemy is..."), keep AI in check, not letting him build up an army and DoW you but instead you pick him off continually and heal, territorial expansion, science/gold/cultural growth by way of puppets, you pay maintenance on your army anyway so might as well use it, extra GG's for citadel bombing or reliquary/mausoleum bonus, pillage gold, lots of workers from steals, archaeologist/missionary/prophet steals, CS liberation ...
Benefits of 100% peace? Different diplomatic benefits, trade, slightly smaller army, swords into plowshares, faster game time (in real-life time), don't have to watch out as much for dumb stuff like his scouts killing your generals or caravels on your trade routes. I'm sure I missed stuff, but anyway the list is shorter.