My computer at work has multiple domains set up.Why is this done?Are there any benefits?

Can you be more specific. What do you mean by  "multiple domains setup".

Any computer can be joined to one domain and one only.

Hi

If you mean that the picklist got more than one domain, it can happen if thrust are done between domain. You just have to select the good one for you.

Hi,
 
You may reference the link below for detailed information about multiple domain:
https://technet.microsoft.com/en-us/library/cc780856(v=ws.10).aspx

Best Regards,
Eve Wang

Some reasons to create more than one domain are:

• Different password requirements between departments or divisions
• Massive numbers of objects
• Decentralized network administration
• More control of replication

Although using a single domain for an entire network has several advantages, to meet additional scalability, security, or replication requirements you may consider creating one or more domains for your organization. Understanding how directory data is replicated between domain controllers will help you plan the number of domains needed by your organization.

long story short: If you have a small organization without any specific reason its not optimum to have more than one domain.

My computer at work has multiple domains set up.Why is this done?Are there any benefits?

Hello,

this belongs to trusts created with other domains/forest so you can access resources there or they can access resources in your domain.

For details you have to talk to your domain admins.