Here's why:
* Prepositions show the relationship between a noun or pronoun and another word in the sentence. They often indicate location, direction, time, or manner. Examples include "on," "in," "at," "to," "from," "with," etc.
* "Bought" is the past tense of the verb "buy." It describes an action.
Let's see an example:
Incorrect: "I went *bought* the store."
Correct: "I went *to* the store."
In this case, "to" is the preposition showing the direction of movement.