If you don't have a micro-SD card that you can write to, you miss out on the following...
a) Logging your tracks to the card. This means that you have the "active log" (your current track), and simplified saved tracks that have had all time and height info removed, and half the points thrown away. With the card, you retain full details of your rides, down to a point every second for a week or three, depending on card size. Without, you have to download the active log at the end of every day or lose information.
b) User loaded points of interest - you can't download all the GBG pubs from CAMRA, and use the "find" button to take you to your nearest decent pub.
All waypoints, routes and tracks that are used for navigation have to come from internal memory, and are the same regardless of whether you have a card or not.