What is Sequential ? Sequential simply means that it is in order so in the case of the gearbox the gears are in order.
What are the benefits of sequential shift ? You will get the next time every time unlike a H-pattern that can select the wrong gear on the gate. The confidence in this will allow to to shift quickly without taking your attention off driving.
Is there a benefit to sequential shift on a synchro box ? You will get all the same benefits and time savings per shift on a sequential synchro box vs H-pattern as a Sequential dog box vs H-pattern. There is always a guy arguing this with us on the internet but to be blunt he has no idea what he is talking about.
Is sequential just for race cars ? Although you will appreciate the benefits the most under race conditions the sequential shifters are still nice to drive with day to day.