Much of this likely comes from two places, one linked to a health reason, one linked to convenience.
First, it was thought for a long time that grains were the correct first food to introduce to a child, then vegetables and fruit, then much later meats. Additionally, breastfed children need a source of iron at around six months. As such, cereals for infants are typically iron fortified, and at least one serving must be eaten every day to get sufficient iron. However, these cereals are pretty boring, and early on the child is likely not eating very much anyway - so if you want to introduce fruits, AND give him/her the iron he/she needs, you mix that fruit with some iron fortified cereal.
Second, adding fruits to the cereal makes it sweeter, which makes it more likely the child will eat it. Just like many adults would prefer oatmeal with strawberries or blueberries, so would a child prefer grains with fruit. Again, if you're starting with grains, this is a reasonable way to encourage consumption of grains.
Nowadays, there is no longer belief that grains should be the first food your child eats, so this may be less important. They are still reasonably easy first foods, and a reasonably easy way to get the needed iron, but many parents do not start with them any more (as they don't particularly appeal to many kids). However, there's no particular reason to have grains if your kid doesn't like them; you can get iron drops, or even introduce meat fairly early (very finely purée the meat) as we did with our second child, who hated grains. Once you have introduced enough foods one at a time to ensure no allergies, you can give your baby complex meals similar to or identical to what you eat at meals (just make sure to reduce sodium and pointless oils).
The American Academy of Pediatrics has some fairly good suggestions for starting out. They generally recommend starting with whatever you want and the baby seems to like, so long as enough iron is consumed. This page does a good job of digesting their recommendations.