Cause Delibird's been up all night workin
Both in the anime and in the games, Delibird serves to deliver packages or presents, and being based off of Santa Claus, it often has a world-wide operation. And much like Santa Clause, while everyone's asleep, his work is done. Santa Claus does not have time for sleep, an neither does the hardworking delibird. Delibird must stay up as long as possible to deliver as many presents as possible to as many people as possible, so it the real world it has Vital Spirit, to keep it awake and keep it motivated. This is further proven by it's other ability being hustle, reflecting on the fact that Delibird has to hustle to get its work done. Delibird is so laborious in fact that even in it's dreams, even in a world of dreams where anything is possible, Delibird continues to work.
So, canonically, there is a purpose, but there is none competitively speaking.