The six-month validity rule
Many countries require your passport to be valid for at least six months beyond your date of return. Even where the rule is shorter, airlines enforce it strictly, and a short-validity passport can mean being denied boarding. Renew early so the rule never catches you.
Blank visa pages
Some visas and entry stamps need blank pages — often two facing pages for a full-page visa sticker. If your passport is nearly full, renew before applying rather than risk a refusal for lack of space.
Renewal before a visa, not after
A visa is linked to a passport number. If you renew after a visa is issued, you may need to travel with both the old and new passports, or get the visa transferred. Renew first, then apply for the visa on the new passport, to avoid this entirely.