We use prepositions to tell our readers the relationship between nouns and pronouns. Imagine that you write:
On Tuesday, I put the gifts under the bed next to my slippers.With a few simple words you have given your reader a clear picture of when you performed an action (on Tuesday) and where you acted (under the bed) in relation to another object (next to your slippers).
Using a preposition requires an object. How can you explain the connection between two things if you don’t specify which two things are related? Prepositional phrases act as signposts for your readers, showing them how the parts connect and interact (or don’t, as the case may be).
Like other parts of speech, various prepositions illustrate specific relations. You can fall against, into, or with something and each of those communicates a different experience. Some are number-specific, as between requiring two objects, no more and no fewer, and among requiring at least three. Thus, you can be between a rock and a hard place among your peers but not vice versa. Learn little words, like prepositions, as well as the more impressive ones when you expand your vocabulary.
Under certain circumstances, especially in casual conversation, you and your audience assume the prepositions or objects rather than expressing them. Had I begun that sentence with “The other day,” you would assume that I was acting “on” that day. We understand that preposition to apply even though I did not explicitly state it. Indeed, some formations are assumed so often that you would sound foolish or foppish including them.
Finally, we have the big debate about ending your sentence with a preposition. I already had my lengthy rant on that subject, Part One: What Do You End a Sentence With? and Part Two: With What Do You End a Sentence? Even if you feel that using that ending preposition is acceptable through common use, it creates one more reason for your reader to miss your point by focusing on your individual words.