In my Pathfinder Society game I'm playing locally, I'm doing a Human Fighter who specialized strongly into grapple, but at level 4 (or sooner) I'll be multi-classing into Wizard. Which is ridiculous since I got in full plate, but Wizard gives me spells like Animal Aspect Gorilla which grants a massive +4 to grapple rolls, and Bull's Strength grants +4 strength. Not to mention the King Crab familiar, which grants a passive +2 to grapple. So despite how much of a bad idea this is, at simple level 1 of wizard I get a +2 to my CMB for grapple and that alone makes it worth the level.
So what, you can't pick a good multi class for your barbarian? Get creative man! Barbarians don't wear much armor, if any, so they're prime for light spellcasting. Even level 1 gets you an unlimited number of 0-level spells and like 2 1st levels. Get an item of enhanced intelligence and you can push that further, and wizards also get a selected school of magic and get +1 prepared spell per day to that school, of every rank they can cast. IE - prepare 1 more 0 level, prepare 1 more 1st level, but can't prepare any 2nd level unless you have the Int to support an additional 2nd level spell, or a high enough wizard level. I mean, Shield is a 1st level spell that gives +4 AC. Mage Armor is another 1st level that gives another +4. 12 Int gets you +1 1st level spell per day, and chosen school grants another +1 for wizards. So right there, a level 1 wizard with 12 int can cast 3x 1st level spells.
There's several classes where 1 rank into multi is a great help.