In every NFL draft, there are many trades. Some trades are huge blockbusters involving many players and/or picks. Others are small, but they all impact every pick that follows.
The fundamental problem with mock drafts is the lack of trades. Trades lead to fewer teams reaching for needs, and fewer good players falling.
On draft day, we frequently see NFL teams surrender an arm and a leg to move up, and this high price motivates teams to move down. Last year, the Cleveland Browns traded down from No. 6 to No. 26, and someone will make a similar move this year.