If you were to take away the option to make something writable, it takes away part of the reason for source control. If a user has a file checked out, and another user on your team attempts to check out the same file/asset, the user does receive a warning saying it is currently checked out and asks if you would like to make it writable. Basically it means two people should never be working on the same checked out file/asset at the same time because only one is going to be implemented into the main project and checked back in as the usable asset.
In other words, if I were working on a checked out material and you attempted to check out the same material, the file within perforce will be locked/grayed-out. This means another user is currently working on the asset and making changes to the asset might be overridden when you attempt to submit changes to the same file. There are some cases in which you might want to have the same file checked out, and in that instance you would want to make the file writable. Working with Source Control is a lot about organization and communication with your team so you can work on multiple aspects of your project all at the same time and combine them into a single unified master project.
Be sure to double-check any files or assets you are editing while connected to source control as it can cause file discrepancies and corrupted assets if not done correctly. If you have any other questions please do not hesitate to ask.