Frodo was brought along because noone else was either willing to hold the ring, or capable of withstanding its allure. Bilbo was brought along because gandalf is a very strange old man that apparently is able to tell that a fat tiny gentleman of leisure would make for an excellent hero with a bit of experience.
Maybe Gandalf had taken a look at Bilbo's character sheet and noticed that he'd rolled great for his stats.
Plus, you gain XP SO fast when you're a 1st-level character in a higher-level party. Just so long as you can stay alive the first few combats . . .
I'm the author of the Alex Verus series of urban fantasy novels. Fated is the first, and the final book in the series, Risen, is out as of December 2021. For updates, check my blog!