The PT is used to penalise micro or picocells, so that (fast) moving MSs do not reselect those cells.
Consider an MS on a highway being covered by a macro cell. If this MS enters the coverage area of a micro cell, it will try to reselect this cell. After it moves out of the micro cell’s coverage area, it will then have to reselect the macro cell. To prevent this, the PT of the micro cell can be set to eg 15s, to ensure that the MS reselects this cell only if it has stayed in its coverage area for more than 15s. That’s how the PT is used.