Think some of it, the bad part, is a commercial drive to go the "no expense spared" route.
But part of it is good, and it's about the fact that basically, usually, it's the bride who has to deal with all the pressure and hassle and work of dealing with organising a big event. So it's a reminder that at the end of the day, it's ok to make some choices that suit the bride, have some things that make her happy.
Yes, some people have helpful mothers/sisters/friends/partners/wedding planners, but usually, most of the organising is down to the bride. And some people have very stressful/picky families and friends, or tight budgets, or all manner of other extra stressors. So sometimes when a bride is stressing over what choice to make about some aspect of some thing, kind friends say "it's your day, choose what you would like".
I used to organise events as part of my job, and I found planning my wedding much more difficult and stressful than anything I did professionally, so good knows what it like for people who don't have some experience in it. Some people enjoy that kind of thing but a lot don't.
Some selfish people take it too far and go all bridezilla, but in my experience they are selfish people anyway, so it would go that way anyway. The "it's your day" thing is for the person who will be up arranging flowers and putting together favours at midnight the night before, stressing about whether mil/distant cousin will get the right meal and so on.