He was attempting to capture Shrek or Fiona. After making their deal, he received the kingdom of Far Far Away. Sooner or later Shrek would reappear and have his chance to use the escape clause in their contract. Rumpelstiltskin was preemptively capturing ogres to make sure that the two would never be able to kiss.
Being sorcerers, witches possess powers beyond the natural. Also, not only do their flying brooms allow them to move around quickly, they also allow for a bird's eye view, thus making it easier to spot and capture ogres.
Shrek Forever After is the fourth movie in the Shrek franchise. The first three include Shrek (2001), Shrek 2 (2004), and Shrek the Third (2007), all of which are based on characters from a children's picture book written and illustrated by American cartoonist William Steig.