"Send to" is really the plain verb "send", with a destination or recipient specified using the word "to". You normally need a direct object as well, which is the thing you send. E.g. "I sent a letter to my brother" or "My office sent me to China". "Send for" is a phrasal verb meaning "summon". (The idea is that there is some sort of messenger who is sent, or at least some intermediary who is involved, but their identity is not important.) E.g. "The king sent for his chamberlain" or "The family sent for the doctor".

These words use depend on the type of context. For example: I send a gift for Maria. I send a gift to Maria. In the first sentence (for), there is a person or medium through which the gift flows. But in second one, it is little bit direct. It is just my interpretation.

send someone to somewhere or someone be sent to somewhere you send for someone : you want to find out someone