Some people have baddies on their credit report, and they will be stuck with them for 7 years. It can be advantagous to add new, clean tradelines to 'dilute' the unclean ones 
And if one clean addition is good, several clean additions is even more so, right? Well, maybe. And if all they can get are store cards and gas station cards and secured cards, well, at least the store and gas station cards don't require deposits.
Then there are people who see good sign up bonuses, like the 60,000 points I'll be getting from my Citi Premier card. But, adding a new card will really reduce your average age of accounts, unless you have lots of cards because then the average of say a dozen cards each many months old plus one card that is 0 months old, is still a large average. In this case you really don't want a large number of store cards, instead you want cards that actually have advantages for you (5% cash back, sign up bonuses, or a card from a company that allows you to combine credit card limits, etc).
In my case, I have many cards that give me 5% cash back on just about all my spending categories except Drug Stores, Rent and Utilities (other than cell phone). I now have 2 cards with $30K limits, one of which is an American Express. And I have a card that doesn't have great benefits at all, but is from a bank I'm planning to use a lot, so I decided to also get one of their credit cards (rather than using their debit card in an emergency). And I have a card with a really nice sign up bonus, that barely dented my average age of accounts
Basically a lot of credit lines on a credit report makes for a stronger report. Just as experience with large credit limits over time without carrying balances demonstrates that you can handle those types of limits. You don't have to do any of that, but since you are reading at least one credit card information site, you're likely to encounter the occasional great sign up bonus or great cash back opportunity, so why not be ready to take advantage of them?
