Originally Posted by streether
EDIT: Sorry, just realised this should go in the Q&A forum but don't know how to move it?
Hi and welcome
At the top right of your post you have a report button (small triangle with exclamation mark), click and ask to move your thread.
Done for you
For the rest, at the beginning I would say to get used to your new nexus 5. Mostly if you want to "customize" your phone the root will be necessary at some point. With a custom launcher (Nova, Apex for example) you can change the look of the icones on homescreen/app drawer, change the number of homescreens, change the number of rows/columns but if you want more (full theme) you'll need root (Xposed Framework with Xposed modules for example that allows almost 100% customization for example
, custom roms, etc...).
Root allows access at the files system (in short) so you can modify a bunch of things..., without root about theming you'll be limited.
[HOW-TO] The Nexus 5 All-In-One Beginner's Guide
[GUIDE] Nexus 5 - How to Unlock Bootloader, Install Custom Recovery and Root
It depends of what you want exactly...