One other point about breasts: they can play a big part in determining our 'passability' (whether or not passing is important is another subject).
Peoples' initial impressions of ones gender are based on a number of cues, of varying importance. A low voice, being tall, wearing pants, having large hands and feet, short hair, facial hair, and a flat chest and hips are all seen as male traits. Long hair, petite size, makeup, smaller features, skirts or dresses, high heels, and a curvy body are female traits. Most people show a mixtue of male and female cues, but the preponderance of one or the other is what determines our impression of someone.
For example, Bea Arthur has a very low voice, but no one would take her as male. But if she were flat chested, had a crew cut, and wore jeans and a t-shirt and no makeup, that impression might change.
A young MTF transsexual, with a petite body, small features, and a flat chest can still pass as female fairly easily, even dressed in jeans and a top. An older transitioner, 6 feet tall, with big hands and craggy features, needs more help such as a skirt and heels, makeup... and breasts. More female cues are needed to outweigh the male ones, and breasts are a very strong cue.
We can't change our height or the size of our hands & feet, and modifying our facial features is extremely expensive. But breasts can be changed, with hormones, implants, or a few dollars worth of silicone rubber.
They're one of the few things we can change, so it should be no surprise that, to those of us that need them, they are a very big deal.
