Use of "le" on the end of the infinitive

Use of "le" on the end of the infinitive
 

¿En qué puedo servirle?

That sentence means "how can I help you?"

My question is: why is "le" added to the end of "servir"? I need help with the rule on that. Can you give me some other examples of when to use "le" at the end of a verb?

Thank you,

Pronouns are tacked on the end of the infinitive, unless you're dealing with an auxiliary verb, like querer, poder and deber. You then have the choice of placing the pronoun prior to the conjugated auxiliary verb or suffixed to the infinitive.

Quiero hablarte. = I want to speak to you.
Te quiero hablar. = I want to speak to you.
¿En qué le puedo servir? = How can I help you?