It is a very open question. Optimizely is a great AB, MVT and Personalisation tool. that is the way one can develop and optimize user experiences and key site covnersion metrics.
Your optimizely consultant / sales contact can tell you more particular to your site.
but I can think of some Pros
- You can test new variations / build and test experiences / site changes before asking/spending developers/resources
- Once you have a winner variation / test you can push it live straightaway and needn't wait for devs to build it.
- You have a community here where you can get loads of ideas to test and build great experiences
- You can increase conversion and can also get PROMOTED
- it might make your site a bit slow to load sometime. but please ask your tech leads to inspect the snippet.
Hope that helps !