Gaining knowledge is not a mandatory thing, it's an interest and varies person to person. Since SAP is an ERP and all modules are tightly integrated, it always helps if you have some basic knowledge on each module. And it's again depends on your interest and requirement.
Nothing is mandatory, but if you start off by setting up limitations and blocks for yourself, you are never going to be as successful as someone who does have that other knowledge - especially when it is pertinent to what you will be doing. It will help you at the most basic level communicate with your development team.
