Why did Buddha use the 'Raft Simile'? The Raft Simile says, in part:


  I have taught the Dhamma compared to a raft, for the purpose of crossing over, not for the purpose of holding onto. Understanding the Dhamma as taught compared to a raft, you should let go even of Dhammas, to say nothing of non-Dhammas. [MN 22]


Why did Buddha use the 'Raft Simile'?
This Pali Canon parable is one of the two most popular images among Mahayana students (the other one being the parable of toys used to lure children out of the burning house).

The other shore is Nirvana and the raft is the concepts (of Dharma). The concepts are used to get across, but the final destination is beyond concepts. Rafts don't move on dry land, sun rays have nowhere to fall if there's no ground. Concepts don't hold in Emptiness, they have nothing to hang on. 

The whole point of Buddha's teaching is liberation of mind. The entire appearance of samsara with all its problems and sufferings is a giant self-perpetuating complex of concepts. Our problems are concepts in the space of relative definitions. To be a master of concepts rather than victim of, is what the practice is all about. Being a master of concepts one can create and destroy concepts as needed, without being bounded by them.

Unbounded, vast, limitless, open - is the other shore. The raft of Dharma is a vehicle, not the destination.