2003 SAAB 9-3 repair Question space saver spare

Tires and Wheels problem
2003 Saab 9-3 repair Question 4 cyl Front Wheel Drive 33000 miles

I had a flat rear passenger tire. I changed it with the space saver spare tire. The wheel locked up and would not roll. I bought a new tire to replace the flat tire. When I put the full size tire tire on, it rolled fine. Is this normal? Why put a space saver tire in the trunk, when iy doesn't work?
If you could shed some light on my situation, I would be very appreciative. Thank you.