Eradication is really hard. If you just kill mosquitoes in a certain area, what’s gonna stop them from coming back? You’re just not gonna get all of them.

This way you introduce a mutation that can actually propagate through the gene pool, disabling the undesired trait for future generations. It’s also highly selective, so that you don’t accidentally get rid of other species or poison an area with pesticides.

Also living beings have no “purpose”. They fill an evolutionary niche and shape the ecological system around them to a degree.