A simple way of saying this is: Christ used comparative terms to indicate and pinpoint those who either have great faith or weak faith. Faith is not a sort of pill that we take and have it consistent throughout our lives. It is something that we need to work on. When we know and understand who our object of faith is, then we would work genuinely towards growing in faith.
We also need to grow in faith because all over the world there are Christians who struggle in their walk with Jesus Christ. The reason for this is we doubting Him. We doubt Him because of several reasons; whatever those reasons might be, we can still strive to live a life that we will not doubt Him; thus giving Him all glory.