While there certainly are parts of the password rules - like length, complex syntax, special characters, etc - that indeed may contribute to improving password security, the often stated requirement to change passwords every 90 days has far less obvious benefits.

There are four basic ways for a bad guy to get your password:
(a) Ask for it. So-called "Phishing" and "Social Engineering" attacks still work, and always will
(b) Try dictionary words at the login prompt in the hope to get lucky ("Brute Force")
(c) Obtain the encryped/hashed password somehow, and crack it
(d) Leech the password off your computer with keylogger malware

None of these four scenarios becomes less likely if you change your password every 90 days. If the bad guy can't break the password hash (c) within a couple days, he'll likely just look for an easier target. Attack (b) is also out for quick wins - either it works within the first couple dozen passwords tried, or the bad guy moves on to easier prey. If (b) or (c) are successful, or the attacker already has the password through (a) or (d), 45 days on average is more than enough to empty out your bank account or use your email address for a big spam run.

The concept of password expiry remained the same for the last 25 years or so. Infosec professionals, auditors, PCI, ISO27002, COBIT, etc all keep requiring it, unchanged, even though the threats have changed quite a bit. Forcing a user who had a weak password to change it will just make him pick another weak one. Forcing a user who had a very strong password to change it will eventually annoy the user into using simpler passwords.

So what gives? There is one practical benefit. If someone has your password, and all they want is to read your email and remain undetected, they can do so forever, unless you eventually change your sign-in secret. Thus, regularly changing the password doesn't help much against someone breaking in and making it off with your goods, but it DOES give you a chance to shake off any stalkers or snoopers you might have accessing your account. Yes, this is good. But whether this benefit alone is worth the hassle and mentioned disadvantages of forcing users to change their password every 90 days, I have my doubts.

Infosec risk management is about identifying threats and vulnerabilities, and then picking a countermeasure. But if the chosen countermeasure doesn't in fact make the identified threats less likely, all we do is play security theater, and the countermeasure is one that we don't need.

Unless, of course, "best practice standards" and audits force us to have it.