A conventional wide-angle lens (in contradistinction to super wide-angle lens) comprise an area from 24 mm and 35 mm, it is an absolute must have if you are going to take pictures of landscapes. Wide-angle lens are also common for surroundings-portraits (for example that kind of photoshoots about famous characters, politicians etc. where you see a lot of the environment).
If you want, for example to photography a firefighter in a fire station, you should take the wide-angle lens to get parts of the fire engine on the picture. Wide-angle lenses are also good to get an overview about something – draw near to a situation. There are wide-angle lenses with 24-70mm or 11-24mm.
I take this kind of lens if I’m taking pictures of…
…landscapes with a camera which doesn’t have Full-frame digital SLR.