Most perennials and bulbs bloom for 4 weeks or less.
A garden designer can help you plan for bloom rotation, so that you have something blooming all of the time.
A good designer can create a series of stunning combinations, so your garden has a different look every 4 weeks or so, and colors don’t ever clash, and you don’t have dead times in the garden when nothing is blooming.
A terrific designer can plan a garden that is at its peak during your annual summer party.
The very best designer is Mother Nature, and she gleefully trashed the work of all of those delusional human designers by throwing out a spring like this one.