They tell us important information about something we want: where it is. Here’s an example. Imagine you’re travelling in another country.

Tourist: Excuse me, where is the bathroom?
Resident: It’s behind this building, next to the coffee shop.

If you aren’t familiar with words like behind or next to, finding a bathroom might prove to be more difficult.
Even simple prepositions like in, on, and under can be helpful, and sometimes funny.

3歳児クラスでおもしろい出来事がありました
Recently in a class of toddlers, we were finishing a song with, “one little finger… put it on your… nose.” 6 children had their pointer fingers where they should be, touching their noses.

But one little boy had his finger securely inside his nose.
“No, no, no, on your nose.” I said, “not in your nose.”

I’m certain he understood the meaning, but when you’re little and your finger is so close to your nose, sometimes it’s hard to resist the temptation.

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乗り物に関する前置詞の一例
Here’s a quick tip for prepositions of location related to transportation: Why do we say “in the car”, but “on the train”?
Generally we use on for public transportation. A good rule of thumb is if you can stand up inside the vehicle, we use on.