Why do some people feel the need to say, “Know what I’m saying?

They are not confident of their own or others' communication skill. They are always scared of being misunderstood. Hence they use such words or repeat their words often. This also becomes a habit.

May be because of some habit which has grown among them over a period of time or may be due to any other reasons but the fact is that it is very irritating...

Well i havent heard any person saying that but according to me it must be some habit and they may even use it without any intention.

May they have a doubt whether they have taken thier information in the correct way and fear of being misunderstood.

because they want to let understand their meaning to others in indirect way

Using same words becomes a habit. I know one man who cannot talk without saying- " i mean to say that...". another person always says- "As you know.." Once, before he could say more, I said- I do not know. after this he became caution and tried not to use these words.

It is like mannerism. that is got habituated from child hood in all languages they say not only english

It happens when we assume that the person to whom we are talking is not getting the point about what we are talking then we normally used to say that do u know what i m saying.