Hello Closetnaturist
I have a mountain bilke and a road bicycle, the racers(road bicycle) tyres are very small tyres which you can't use off road but if you have a mountain bike you can just change the tyres, you get nobbly tyres for off road and slicks to use on tar roads.
I prefer my mountain bike which I use off and on roads but I don't change my tyres, I just keep my nobbly tyres on all the time.
I suppose it depends what the person prefers to use but you do get different size bicycle frames, if you go to a bicycle shop they will advise you on what frame best suits your body frame, I'm a small build so I use a small frame bicycle.
Hope it helps you out.