I found that both words have similar meaning as in when they are verbs, they can mean similar action. But I'm confused with their usages when this similar usage takes place, which is "sending". May I ask what is the difference in usages between "send" and "dispatch"?
Haste is the difference - dispatch primarily means to send off in hurry. It also has the additional meaning of kill a person. Etymonline has the following:
dispatch (v.) 1510s, "to send off in a hurry," from a word in Spanish (despachar "expedite, hasten") or Italian (dispacciare "to dispatch").
Meaning "to get rid of by killing" is attested from 1520s.
Interestingly, the spelling variant despatch which is equally popular now was due to a spelling error in the Johnson's dictionary.