Barad-Dur Tracker
Minion • Uruk-hai
Strength: 6
Vitality; 2
Site: 6
Garrison- Damage +1. Assignment: Play this minion for free from a fortress; it is strength +1 for each garrisoned minion you can spot (limit +3) until the regroup phase.
Mordor Tower
Condition • Support Area
Fortress 2. Assignment or Regroup: Play a minion stacked here for free.
In answer to your questions, the reason you would use these abilities, it makes more room in your hand for possessions, companions, events, etc
and it has some nice advantages to grant, lots of minions, less roaming time, damage and strength bonuses.
For example, in the Shadow phase you would remove
to play Barad-dur Tracker, either normally, (which would not be advisable, as you would lose his ability) or on Mordor Fortress, for example in which case you would pay his Garrison penalty. In the
Assignment phase, or the Regroup phase, not the shadow phase you can play them for
free since their twilight cost has already been extracted. They then get assigned, and skirmish normally. Any questions?