Drop tubes allow the powder to settle in a better orientation, I guess. You can get more powder in the case, which equates to cleaner burning and more power. Also, the more compact you get it to begin with, the less likely it is to settle and form a potentially dangerous void. The larger the capacity of the case, the more important a compact, compressed load is. You definitely do NOT want any air space in the cartridge. The bullet should compress the powder about an eighth of an inch or so.
Since you should be loading pure lead for BP loads, it helps to pre-compress the powder (even after using a drop tube) using a compression plug in a reloading die so you don't deform your bullets.
No case sizing necessary when reloading BP catridges, unless you run up against chambering problems when using different guns. Otherwise, you will get better accuracy if you don't size. No neck tension is needed, and the powder keeps the bullet from seating back in the case, and the crimp keeps it from moving out.