Why use `thread' as an argument of Syscalls?

Hi all,
I find that FreeBSD Syscalls always have an `thread'
argument, for example, preadv(/sys/kern/sys_generic.c)
has a `td' argument. But some Syscalls may rarely use
this argument, and thay ( and functions they invoke) can
get the `thread' who make the Syscall _easily_ via
`curthread' macro if needed. So the `thread' argument
seems not needed.
Can anybody tell me why use `thread' as an argument
of Syscalls?

Thanks.

Regards,

--
Three passions, simple but overwhelmingly strong, have governed my life:
the longing for love, the search for knowledge, and unbearable pity for
the suffering of mankind.
---------Bertrand Russell
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