I just got a new job at a medium-sized (~100 employees) company and one of the first things I was told is that I cannot use my own computer, because I need to be able to connect to their network, access files, etc. I didn't think that made much sense because to my knowledge, as long as I'm on their network, I should be able to access anything I need to.

So I asked my friend this question, who told me it might be a security thing. Could there be a security-related reason as to why I'm required to use my employer's machine?

So this is an interesting question with a few points into why you not only should WANT to do this, but should do this for your own safety and security. It helps first if you understand that companies point of view before we talk about how it can benefit you.
Why would a company want to do this?
Many reasons. It makes it assured that your computer can access the network, do what it needs to do, and function how they need it to at a baseline. This way the IT department can maintain it easily, quickly, and up to standard.
Can they make me do this?
YES THEY CAN! They are having you work on their property, with their property, to make sure it works properly. This way you can actually do your job.
Should I do this?
Oh god yes.

This lets you pass the buck if needed. Now if something that is supposed to work, doesn't work, it isn't your fault.
Maintenance becomes a breeze because if your files are backed up to a safe place (any installers you used as well), then if something really bad happens they can restore it to a disk image and have your computer back to you in the matter of a few hours instead of days.
IF you leave the company for any reason, you don't have to relinquish your personal computer to them for driver scrubbing or making sure you don't take any company software or intellectual property with you.
IT has no claim to touch your personal computer for any reason.
For security reasons, you can make sure that your work computer is up to their standards and any potential breach won't be your fault, but their bad policies' fault.

And here's the whammy: It keeps you safe from the company!
In using the company computer your own personal information won't be on the company network, and you can keep your private life away from your work life. This is a big advantage because you can make sure that your own data, is your own data.
