For safety precautions. This way you know what the heat tapes temps are exactly. Putting the probe in a tub will only cause the heat tape to work harder and get hotter. If that probe were to move, the snakes are at risk of being burned.
I dont use Herpstat for my racks but Johnson. All are set at either 89,90,91 and i get those temps exactly in room temp 80-81. When the t-stat kicks on the heat tape does read 100f+ but inside never goes above the set temps.
On the herpstat check the settings on herpstat to be sure its working to its full potential.