We have a Boori Ranch Cot with solid ends and slatted sides that remove to convert it to a toddler bed. In my mind the idea of the toddler bed is that it's lower to the ground than a usual single bed and it's smaller and more snug. The other main advantage is that it doesn't take up as much space as a single bed. For us that's a real issue when the room needs to have a bed, change table (a necessity given the fact that I have a bad back), a chair for stories (more comfy than a bed for us) and a wardrobe.
We kept the sides on the cot til my ds was 2.5 years old and he climbed out. We converted it to a toddler bed that very afternoon which was great that we didn't need to urgently go out & buy a bed when he climbed out of his cot. From memory we didn't have a safety rail that first night, but I bought one the next day - just a standard safety rail from Target in the baby/kids section. It did go most of the way along the toddler bed edge, but it worked well. He never fell out. He was happily in that toddler bed til the age of 4 when he got a single bed. My dd is now in it as a cot and will be for a while yet, so we're definitely getting our money's worth from it.
My mum had one of those white toddler beds from Target. Again mainly for safety (low to floor, built in safety rail) reasons, but also for space saving reasons. Worked really well.