I own an upright exercise bike and love it (I have a nice quiet one so I can watch TV), but found that I couldn't sit on the seat comfortably for quite a while after my baby was born. (Go figure!) It was hard to get out of the house with the baby because it was the middle of winter. My portable exercise bike saved my sanity! I could sit in a nice padded chair and, like EdRick mentioned, blow off some steam.
When the baby was a little older and the weather was a little better, I joined a gym with childcare so I could take some classes, run on the treadmill, and generally get some real exercise. But for that period when I was allowed to exercise a little but couldn't leave the house or sit on my bike, the portable bike was great.