I was talking about seat covers that you make yourself. In the 60s and 70s, there was cotton content in automotive fabric, and vinyl was stiff and hard. Now days, there is no cotton content in fabrics used for automotive use, they are all 100% synthetic, and vinyls are vastly improved and are soft and supple. Steaming fabric with no cotton content is a waste of time, as is steaming a vinyl seat cover. Both of them will just get wet. Seat covers, headliners, and convertible tops that are sent to you folded up and may have been in a box for months are a different thing. Those things may need a little help and steaming or heating them makes sense.
As for seat covers you make, if you use quality materials, and your workmanship is good, you don't need to heat or steam them to get them to look good. If it doesn't look good, my suggestion is to take them off and find out why it doesn't look good. Heating or steaming will not fix poor workmanship, nor are they substitutes for good workmanship.
No one lives forever, the trick is creating something that will.