2) Protection from branches, rocks and other objects that you might brush up against while riding (even if you don't crash/fall)
3) Foot protection and ankle support when you put a foot down in a sharp turn.
4) Abrasion / impact resistance when you do fall and your leg is between the bike and the ground.
In my opinion, boots are a must. Anything is better than nothing, and you quickly get to a point of being adequate. Beyond that are issues of comfort, durability and fashion.
I tend to end up wearing the boots all day so they need to have a basic level of comfort. If your bike breaks down you may be walking back to camp so a reasonable fit is important. Buckles that break or won't stay closed can be really irritating.
No one mentioned broken tibias ??? You should wear knee and shin guards , but boots also have those nice plastic things in front too. Its hard to start out, but the gear is a big deal. Helmet, goggles,knee shin guards and boots, gloves, chest/back protector.
Get them and use them :ride: