I have came across a sentence made without "for" or "to", therefore I have a little confused.
The sentence is:
It can help you maintain a healthy weight.
Don't we need to use preposition before "maintain", such as "to" or "for"?
The sentence is perfectly understandable to a native speaker either way. You can use to between you and maintain, but it's not required in daily use.
In my personal experience most people phrase the sentence as you've written it, omitting to when speaking.
This sentence uses the infinitive form of the verb maintain. In English, infinitives can include "to" or they can omit it, the latter is called a "bare infinitive."
In most cases, there is a correct preference for either the bare infinitive or to-infinitive. You have found one of the rare ones where either is correct.