It's just to illustrate how "pitkä" works, I guess. That if a person is tall, they are "pitkä", and if something is long, it's also "pitkä". However, if something is very high, like a building or something, then the word "korkea" is used.
You can also use "suuri" to imply height/tallness, but it does not directly "measure" these. If you say something or someone is "suurikokoinen" it/they just might be very wide, or bulky, or muscular etc. not necessarily high or tall although that's also possible.