For me also it's not a replacement but compliment. I use it during the winter or inclement weather (I'm a very fair weather cyclist!) just to get some kms in and usually do a lot more work than I would do outside. Also I get back pain when I ride outside, not so much inside.
Also as other say, sometimes it's easier to motivate yourself to do an indoor thing than to get everything ready for outdoors and then deal with weather, road surface, traffic etc.
The payoff is when it's (briefly) nice and sunny I can go outside and cycle around various terrains without too many problems... unlike one of my somewhat less fit friends who tend to struggle a lot more.