Why Steemit allow the dust vote, Like votes less than 5 SP.

I am just wondering what purpose do vote serve when they are not able to generate even .001 SBD.

I think we should not allow such low level vote in which the effective SP by user is voting come less than 5 SP. If Steemit do not allow (or suggest user to have SP have more 5 for operation) they why it allow user to upvote with less than 5 SP.