Other funky reasons: long exposure images through small slices of the sky can have less noise than "larger" chunks nearby, that are moving. Nearby objects need to deal with things like the zodiacal light, gegenshein, astroids, the glare of the Sun and reflections off of planets and a number of other things, as well as the glare of stars, which there are many more of in larger fields of view than 6 arc minutes.
A bad analogy:
this is imaging a gnat through a empty space in a square of a screen door. What you want to see is a bear through Gauze.
2. How can we observe objects 30 billion light years away, but can't rule out another planet past pluto?