Online printing services need printer profiles?

I have been slowly getting more comfortable using LR and would like to start using it for printing, not just cataloging and editing. I am a little confused about the need for printer profiles if I am using an online printer service (i.e. shutterfly ). My understanding is that using a printer profile would help my printer output (i.e. if i was using a home printer) match what I am seeing on my screen. However I've come across threads that write of using downloaded printer profiles for printing at offsite labs - online or even B&M like Costco. I'm confused about this. Wouldnt the Printer need the profile to make sure their output matches my file? Do I need to use a downloaded printer file even to view the photo on my computer correctly?

I am using a PC with Adobe LR 3 with Spyder for monitor calibration. I don't plan on printing proofs at home.