"Buffer" just means it is used between input and output. It does not imply mutability, and many buffers indeed only take their state at construction time and are not mutable.
In my experience, the only functions a mutable string buffer needs to provide are "append string (or to-string-able)" and "undo that append" (which mostly comes up in list-like contexts, e.g. to remove a final comma); for everything else you can convert to an immutable string first.
(theoretically there might be a "split and clobber" function like `strtok`, but in my experience it isn't that useful once your APIs actually take a buffer class).
Considering the functions from this implementation, they can be divided as follows:
Lifetime methods:
init
free
clear
Immutable methods:
print
index_of
match_all
split
Mutable methods:
append
prepend (inefficient!)
remove
replace
I've already mentioned `append`, and I suppose I can grant `prepend` for symmetry (though note that immutable strings do provide some sort of `concatenate`, though beware efficiency concerns). Immutable strings ubiquitously provide `replace` (and `remove` is just `replace` with an empty string), which are much safer/easier to use.
There are also a lot of common operations not provided here. And the ones that are provided fail to work with `StringBuffer` input.