You're right in that to get to the manual select mode, you have to enter sports mode first. On cars that have the sports mode at all, that is. If it doesn't, you enter manual selection mode immediately.
As soon as you start selecting gears manually from the sports mode position, you leave sports mode and enter manual mode. You can't get back without first returning to drive.
Hence three modes:
When towing my camping trailer (1700 kg) on roads with a lot of small hills and curves (think like a typical British road...), I frequently use S mode, not so much as Sports mode, but as Strong mode. It gives a better immediate pulling capability when it changes from downhill to uphill. The more abrupt gear changes are masked by the higher weight, so you don't notice them much anyway.