Homer conceived of the chair as a way of going to the toilet while watching TV instead of having to go upstairs to use the toilet there, which Homer described as "the hardest thing in the world."
While most of the family viewed this invention (and most of Homer's other ones) as "awful," Bart liked the chair.
He was about to use it ("Gangway! Gotta poop!") when Marge stopped him, saying that she didn't want him going to the bathroom in the living room.
However, when Marge was distracted with discussing with Lisa about whether she should have told Homer that the inventions were terrible, Bart managed to use the Lazy Man Toilet Seat while reading the newspaper, also dismissing his family ("Ahem! Do you mind?").
Homer eventually put it to use as a means to take stress off the fact that his Electric Hammer was being credited to Edison's descendants after he accidentally left it at the museum after deciding against destroying a fall-halting chair that Edison developed before Homer developed it ("Let's just say I'm sitting in the right chair").