You do not have to log in to read Wikipedia. You do not have to log in even to change articles on Wikipedia —almost anyone can change almost any article at any time, even without logging in, and many long-time users do not log in. Creating an account is fast, easy, and free, and it is a good idea to do so for many reasons.
They can change semi-protected pages (your account must be at least four days old and have at least ten changes, but these pages cannot be changed by unregistered users).
The use of a username of your choice, as long as it is acceptable and not already used.
Blocked?
Shared IP addresses such as school and company networks or proxy servers are blocked a lot for vandalism which, unfortunately, may also affect users who haven't done anything wrong on the same network. If you are blocked right now from creating an account, we suggest you do one of the following:
Try again after the block on your IP address ends. Go to my contributions and follow the Block log link at the top of that page to find the length of the block. You may also copy the IP address that appear at the top of the IP address's talk page, and then go to Special:BlockList and insert it in the "IP address or username" box and then click "Search".
Create an account while at an unblocked IP, such as your workplace, home, local library, or favorite internet cafe and then log in through the blocked server or network.
Ask a trusted friend on a different network to create an account for you.
If you are blocked, but with talk page access allowed, you can also add the code {{unblock|your reason here}} to your talk page, and replace your reason here with the unblock reason, and you may be eventually unblocked.
Do not create an account to ask for an unblock, or evade a block (make edits even though you are blocked). Doing that would be called sockpuppetry.