Most people use 上面/下面/前面/后面 instead of 上/下/前/后 in conversation. The single character preposition gives a literary/archaic feeling and may confuse the listener.

（However, the combination 左面/右面 are incorrect, 左边/右边 are correct.）

In some cases, using the single character preposition is strictly prohibited in modern Mandarin, like after a pronoun or a person's name:

你后面有一个人。 There's a man behind you.

Here you can't use 后 to replace 后面.

2-character preposition is a modern feature. Classical Chinese always uses single character prepositions. Modern Chinese still has its influence. You can see classical usage of prepositions like 其后 (其 is the third person pronoun in classical Chinese) in writing and occasionally in speaking.