Why is it good to be Thane of Glamis in Macbeth?

The three witches greet Macbeth in act 1, scene 3 as "Thane of Glamis," "Thane of Cawdor," and tell him "thou shalt be king hereafter." Macbeth answers, "By Sinel's death I know I am Thane of Glamis." This is the seat he has inherited rightfully at the beginning of the play. He cannot understand how he is to be also Thane of Cawdor, or king—not knowing that Duncan is later to appoint Macbeth Thane of Cawdor.
The idea is that each position is more powerful than the one before. As Thane, or lord, of Glamis, Macbeth has an authority position within the feudal system of early medieval Scotland. The men who live in the land he controls owe their loyalty to him, and its wealth belongs to him. Glamis Castle still stands today and is considered one of Scotland's most beautiful castles; it now belongs to the Earl of Strathmore and Kinghorne. Glamis is in a highly defensible position, giving its thane a military advantage.
Macbeth describes the Thane of Cawdor prior to himself as "a prosperous gentleman." In becoming Thane of Cawdor, in addition to Thane of Glamis, then, Macbeth would likely inherit this wealth, as well as the additional power, land, and men attached to Cawdor.