The residence permit is not a visa; it's a residence permit. Because it is evidence of your permission to reside in the UK, it can be used to enter the UK. For more information, see https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/457650/Final_v_3_BRP_-_In-Country_information_leaflet_post.pdf.
Relevant excerpts (emphasis added):
The biometric residence permit is proof of the holder’s right to stay, work or study in the UK. It can also be used as a form of identification (for example, if they wish to open a bank account in the UK). The holder is not required to carry their permit at all times, but they must show it at the border, together with their passport, when travelling outside of, and when returning to, the UK.
How do I use my biometric residence permit?
Your biometric residence permit is evidence of your permission to stay in the United Kingdom, and shows the conditions of your stay. It enables you to confirm your identity and your rights to study and/or work here, and to access public services.
You must show the permit to your employer or approved education provider before you start to work or study.
You must take your permit with you if you go abroad while you have permission to stay in the United Kingdom. You may have to show the permit to an immigration officer at the border when you leave the United Kingdom. You will also have to show it as well as your valid national passport or travel document before you begin your return journey, and again when you re-enter the United Kingdom. The permit proves that you are allowed to return to the United Kingdom, but it cannot be used instead of a passport or travel document.