why might a scientist prefer to use the lower powers of a light microscope to observe aspects of unicellular o

Answer 1

prefer to what? TEM? SEM?
anyway, under a light microscope you can watch live organisms, under a TEM or SEM they have to be dead and fixed. Or if you have an enviromental SEM the beam will kill your organism pretty fast.

Answer 2

Because lower powers offer a wider field of view. If you want to see the entire organism and not just an organelle, you need to not be on the highest power. Also, it is sometimes easier to locate a target for viewing at lower power, then move up to higher power objectives.