Why are lobe finned fishes important to evolutionary biologists?
You have clearly gotten this question out of your book so you might want to look up the exact wording of the answer in case your teacher wants you to be really faithful to the book...
Evolutionary biologists are, in general, interested in these fish because these fish led to us. Put more broadly, it was these fish that led to life on the land.
The first kinds of life evolved (we believe) in the sea. The lobe-finned fishes were the ones that first started to come out of the water and to evolve into terrestrial life forms. So I would say that these are important to evolutionary biologists because they form a sort of transition between purely aquatic animals and terrestrial ones.