How do transposition towers in transmission lines work?
The picture shows three common arrangements of wires. I added wire-to-wire capacitor symbols, note that you also have a wire-to-ground capacitance for each wire. Capacitor values decrease as the distance between the wires increases.


Picture is own work, CC BY-SA 3.0

Case 1, Three wires in one level (equal distances to ground, but different wire-to-wire distances):

The capacitance from the middle wire to the two wires on the sides is bigger than the capacitance between the two wires on the outside of the system.

Overall, you want to have an approximately equal capacitance from each wire to the two other wires. Thus, by transposition of the wires, you create, in average, an equal distance (and capacitance) between all wires with respect to each other.

Case 2, Three wires arranged as a triangle (equal wire-to-wire distances, but different distances to ground):

Over the entire length of the system, the distances and capacitance values of the three wires with respect to each other are equal, but the wire-to-ground capacitance is bigger for the wire(s) closer to ground.

By having the three wires swapped over using transposition, each wire spends an equal averaged distance to ground. Thus, the wire-to-ground capacitance values match for the three-phase system.

Case 3, Wires neither spaced equally with respect to each other nor to ground

Now, you end up with two reasons for transposition along the total run of your line.