Newton's first law of motion seems redundant in view of the second law: since indeed F=ma, if no force is acting (and the object has mass), then its acceleration must be , and it must keep its current velocity.

So why is it there?

Newton's laws of motion aren't a description of a natural phenomenom. Forces aren't observable physical objects, except by application of Newton's laws. Instead, the 3 laws of motion can be considered an axiomatic definition of what force is.

The first law just tells you how to observe a force: just look for any object with a non-constant velocity. Now that you know what a force is, you can proceed to measure it using the second law.

Newton was well-versed in the axiomatic method; his 3 laws are a nice example of how it can be used.

Either that, or Newton thought he'd look silly with second and third laws, but no first law to back them up.