man page of syscall says, "it is useful when there is no wrapper function in c library". If wrapper function is available, Is it always better to use wrapper function ? If not, when should prefer syscall over it?
Never.
The only situation where you should ever consider using syscall() is to invoke a system call that doesn't have a wrapper in your C library. It has absolutely no advantages over a libc wrapper, and many disadvantages. (For instance: It is less clear, non-portable, and bypasses compile-time checks on the number and type of arguments.)