A sperate guest wifi network for your visitors and contractors can be massive benefit to the company and here are some great reasons:

1) Segregrates your guests from your main company network. This is one of the main reasons to perform this segregation which leads to an instant increase security and privacy of confidential company data. 

2) Limit and restrict access. If guests athenticate with the guest networking using a shared password or an individual user account , these can be changed without affecting your existing workforce's connection. Access can be limited to a specific timeline for example a week , day or month automatically.

3) Marketing. One of our event companies has used the offer of free wifi to guests as a marketing opportunity where guests are asked to like the company socially. User's signing up to the wifi service accept the terms and conditions for their details to be saved by the company which can then be used to market leads or services using newsletters.

4) Auditing. As you will be providing internet access to an individual through your connection you will liable for whatever they use this for. This Access can also be controlled in regards to what is allowed on the guest network , for example disabling streaming or Netflix as well as enforcing a Web Filter for safe browsing.