Say you've got a fairly competitive Magic: the Gathering deck you really like. It's likely to be tuned to the particular environment it developed in, whether that's what's most popular among your friend group or the metagame at your local FNM (or this month's big-tournament metagame, for that matter -- netdecks are a popular thing).
A lot of MTG play is informal play between relative strangers. It's not "Vintage" because you're not playing with decks designed to win Vintage. You can sit down for some "casual play" at a convention or game store and see everything from an old Lorwyn-era Standard deck to Legacy-grade Countertop to a cross-set mono-green beatdown deck -- and you'll get to play some pretty dang serious games against them.
Clearly this is an environment that favors breadth over really specific answers and hosers and threats. As an example, Gut Shot is a pretty widespread card in M12-Scars-Innistrad Standard because it deals with turn-1 Delver or Stromkirk Noble; it's rather a crap choice against an aggro deck using cards like Loam Lion, Vampire Lacerator, or Goblin Guide, though. Lightning Bolt, in contrast, is a card that is almost never considered useless.
What approach should you take to improve the breadth of a deck for this kind of never-know-what-you're-going-to-face environment?