I have not been able to find anything that explains this marking, but I have two guesses.
The first is that it has something to do with playing harpsichord, perhaps an indication to change register, since Purcell was writing for harpsichord, not piano.
The second is that it is a phrase marking of sorts, an indication of a break. Beat 3 is ending a phrase. The whole measure is a tonic chord, and the two 16th notes are acting as a pick-up into the next measure. In playing this piece, especially on piano, the repeated Es and change in fingering would necessarily mean that the first E would have to be cut off short if the 2nd E were to be played directly on the second half of beat 3. However, because the LH notes on beat 3 are also eighth notes, it is a clear indication that both hands must hold that 8th note for full value and release together. This then means there is a break before the next phrase to allow for the action of playing a repeated note in the RH, and the 2nd half of beat 3 will being "late."