I made it into my 30s without ever having a credit card; the only thing in my credit history was a car loan (which paid off in the minimum time). Then various things caused me to be traveling a lot, and I ended up looking at airline and hotel cards, since I might as well earn miles/points for money I was going to be spending anyway.

My credit report (free from my bank) tells me my score is slightly lower than it would ordinarily be because of how short the card history is. I still get a couple "pre-approved" card offers every week and the occasional offer to raise a limit, but I just shred them.

Why in the world would you want 100k of credit?

Even if you're very flush it's handy to have the ability to use a credit card for a transaction because your checkbook is at home. Maybe you want to buy something and then sell some stock when you get home to pay for it? Credit is handy when you don't need it. I would imagine :)

Speaking only for myself, my total credit limits are huge because my wife likes signing up for new cards to take advantage of special offers and rewards programs. We don't use anything like that much credit, but it's available to us.

Increases credit score, plus I wanted the sign up bonuses. That's only on 5 cards too, btw...

It increases your score because you have a lower credit utilization and you have more available to you (supposedly signaling how responsible you are).

I have about $150k limit on my cards combined. It would be a lot higher but I asked most of them to lower my limits.

I just have a lot of cards. If they want to give me bonuses for signing up I will take them. Free money.

Don't you have to pay annual fees?

Sure but you don't have any obligation to keep the cards and pay fees year after year. When the annual fee is coming up on a card I call up and cancel it. Most cards give you the first year free anyways. Plus the no annual fee cards add up if you've been "collecting" them for a while and there's not usually a reason to cancel no annual fee cards.

Total it would be like $300, but all of them are "first year free".

Plus usually you can call and get the second year free too.

Regardless, I can cancel w.e. and the increase in my credit score is worth it. At the moment if I want to buy a house (for example) I get the best rates on loans

Many cards are first year free but quite a few aren't. AMEX stopped first year free on a lot of their cards. Regardless if I'm getting a bonus I'm subtracting any fees from that bonus to see if it makes sense. If a card is $100 annual fee and the worth of the first year is $500 it's a no brainier to get $400.