Why should one never use the `-a` or `-o` operators with `[`?

Stéphane Chazelas wrote: > a few rules like > > - always quote variables > - never use the -a or -o operator (use several [ commands and the && and || shell operators) > > Make [ reliable with POSIX shells. Why "never use the -a or -o operator"? How can I do "use several [ commands and the && and || shell operators)"?