The Theological explanation
There really isn't one even by their religious definitions.
Since Spencer W Kimball what the prophet is and does has been endlessly redefined, things said by a previous prophet can now be over ruled by a later prophet, nothing any prophet says is prophesy or revelation unless another later prophet says it is or was and even then this can be rescinded by a later prophet still (and bare in mind all apostles hold the title of prophet, but only the church president hold the title of THE Prophet).
"Thus sayest the Lord" comes from the days of the early church and the Journal of Discourses, it is not held to now ( or wasn't when I left the church in the 1990s).
They also now use the terms "Doctrine" and "policy" interchangeably, until someone challenges it at which point the teaching in question becomes policy definitely not Doctrine or in extreme cases "well meaning speculation" by a church authority.

The reality

Ultimately there is only one reason for the prophet to exist today and that is to act as the chief executive for the two "corporation sole" legal entities known as:
Corporation of the President of The Church of Jesus Christ of Latter-day Saints
Corporation of the Presiding Bishop of the Church of Jesus Christ of Latter-day Saints
between which are owned All of the business interests of the the LDS church including their $32billion of stocks and shares, media empire, property empire (Which until the 1920s included a string of brothels) and retail empire.
By law a member of the organisation identified by title has to be the sole titular owner of of the corporation, in the case of the LDS this is the President and Prophet of The Church of Jesus Christ of Latter-day Saints or the longest serving apostle (which is why despite claims of a calling from god, it is ALWAYS the longest serving apostle who becomes the next prophet.)
In short the prophet is a euphemism for the chairman of the board of directors.

@LenHazell53 Wow, the current theology sounds like a "mass of confusion."