It is there for an obvious reason. To quote from this answer,
When you are creating an account to run a daemon, service, or other
system software, rather than an account for interactive use.
Technically, it makes no difference, but in the real world it turns
out there are long term benefits in keeping user and software accounts
in separate parts of the numeric space.
Mostly, it makes it easy to tell what the account is, and if a human
should be able to log in.
A lot of unix/linux security depends on access to files and the right to execute them, and this is managed through user accounts.
So programs need a user account in order to work.
It's common to create a specific account for each application/service/daemon because this gives fine-grained control about what the program is allowed to do (eg don't mess with other programs' files)
However, the main reason to maintain the different system accounts is the compartmentalization for the purpose of security as noted by goldilocks from his comments. As he points out, since the web services are outward facing, the security is an important aspect and the system accounts help in achieving it.