Invitations serve to request one's presence for an occasion. They also create anticipation for that event. Their form and language set a tone. For example, engraved bond paper is the customary device for weddings. A backyard BBQ invite, however, will be far less formal.
Invitations for any occasion convey the following:
Who – host and guest of honor, if applicable
What – event and type (dinner, cocktails only, etc.)
When – date and time
Where – place
A request also calls for an answer. It is important to include RSVP (accompanied by a phone number and a date). RSVP is an acronym for 'repondez, s'il vous plait' (polite French for 'please respond'). You need to know who is coming (how many for seating and food purchase requirements) and by a certain day (lead time for preparation, orders, etc.)
The invitation tells the recipient that s/he is special and that her/his company is desired. The host(ess) is told by the prompt response that his/her hospitality is sought and eagerly anticipated. Thus the invitation plays an important social function apart from the social function. I am a huge fan of invitations!