Comparing BLAST and the Smith-Waterman algorithm
So I got this answer from my lecturer:

Since Smith-Waterman algorithm is based on DP, we will get the best performance on accuracy, but there is a change that the homologous sequence is not with the highest probability (so better matching sequences will be "hidden" behind worse ones).
Since blast is based on the heuristic approach, it overcomes the disadvantage described above, by adding biological information, and therefore make it more accurate from biologist's perspective.