One of the cool things about humanity is that we do stuff that doesn't seem to make much sense, like set sail in a direction without any idea if there is even land out there

The reason to colonize Mars is to see if we can

For a lot of people that's a sufficient reason

And frankly it may be worthwhile because if we can colonize Mars we may be able to colonize a large asteroid full of resources that we need, or something else

Or create a chain of fueling station colonies on planets on the way to a new, habitable planet

Who knows. But sometimes we just should set sail and see what happens