Saisentan significantly affects runner calculations on how dangerous an unrezzed ice could be. If they get surprised by Saisentan and don't have a method to break most of its subroutines, they take a devastating amount of net damage (usually 4-5).
If you are playing Saisentan all of your unrezzed ice look scary early on, enabling high-value early plays. It fights very well against most of the cards a runner might use to break an unknown ice early (e.g. Boomerang, Engolo, and Botulus), so a turn 1 Rashida will probably work out for you even if they do think they have a way inside. It's also costly enough that having an unrezzed ice on HQ could be able to deter an early Diversion of Funds.
After the runner has a killer, it usually costs 3 to break Saisentan, which is not great for a 5 cost ice. MKUltra costs 6 to fully break Saisentan, which is excellent. Note, if they are running Bukhgalter, keep your sentries together, Bukhgalter is awesome against the first sentry but struggles against its friends. (A Saisentan costs 2 for a criminal to break on a server which has no other sentry but 4 on a server which already has a sentry).