Having suited cards when playing Texas Holdem can give you a definite advantage. Being suited allows cards that under normal circumstances might be thrown in the muck to suddenly become playable.
Hands like QTs, JTs, T9s, 98s, 87s, 76s, or even K9s, Q9s, J9s and so can be played in late position when the pot is sure to be multiway, and have a much better chance of winning compared to their unsuited counterparts.
You might be asking yourself, is there really that big of a difference between K9 suited, and K9 offsuit. The answer is a definite YES!
Even if the difference of winning only changes from say 16 percent to 20 percent, this is not just a 4 percent increase, but is a 25% increase! The value of being able to make a flush can turn a loss into a win. Even in pots with 6 or 7 people involved, a flush will usually be good enough to win you the pot. While flushes don't come in often, when they do the rewards can be enormous.
Another benefit to suitedness is having redraws in situations like when you might only have one pair. In these situations, your holding is very vulnerable to someone else making a bigger pair or making two pair or better on the turn or river. Having suited cards can often give you the opportunity to make a flush even when your one pair is drawn out on, and still win the pot.
Another thing to think about in multiway pots is that with more people involved, the likelyhood of one or more of them having suited cards goes up. This puts an unsuited hand at a disadvantage that a suited hand wouldn't face. Playing hands like Q8o or A5o in large multiway pots gives your opponents a big head start if they hold suited cards. This is a large starting gap to overcome that sometimes even the best players aren't able to do.
If you ARE going to play offsuit cards in texas holdem, then make sure that they have big card value. A hand like AQo is affected less by the fact that it isn't suited because it's other obvious benefits make up for the disadvantage of being offsuit.
Basically, if you are ever unsure of whether or not you should get involved, be more inclined to play suited hands than unsuited ones.
HOWEVER - this doesn't mean that just because a hand is suited, you can play it in any situation. A weak hand like J5 is trash regardless of whether it is suited or not. Being suited does improve the strength of your hand, but this doesn't give you an excuse to play bad cards just because they are of the same suit.
In short: Being suited in holdem will offer you a greater chance of making money in multiway pots than unsuited ones, and can make a marginal hand like Q9 playable.