This is a phrase we have all said to another person at least once in our life. And I'm sure it has been said with sincerity and concern for whatever that other person was going through. When someone tells us they're hurting or need support or are going through a rough time, something inside us kicks in and we want to help. But sometimes we don't know how to help or the way to help brings us too far out of our comfort zone. So we resort to saying, "I'll pray for you."
There's nothing wrong with saying this, however we do have to keep a couple things in mind. First of all, we actually need to remember to pray for them! It's one thing to say it - in order to relieve ourselves of any responsibility we feels towards another - but it's another thing to actually do it. Secondly, we have to check ourselves and make sure that saying we'll pray for someone itsn't a cop out for doing something better, or something they actually need. Sometimes they need admonishment, sometimes they need us to walk with them through a situation, sometimes they need more than an "I'll pray for you."
When God puts people and situations into our life He wants to USE us for them. He wants to use our hands and our feet and our words and our actions. We can pray for someone all we want, but just think, when saying "I'll pray for you" you just may be the answer to that prayer.