Equipment adds to the "equipment/achievement bonuses" section on your profile, which adds to your base stats. Some equipment also allows you to hit certain parts of certain ebs. You can read choccy's eb guide to see if a given eb requires equipment to hit it.
is there any difference in behavior between equipments that use percentages and those that use straight numbers? i heard when i first got abyssal sword that it’s percent base will cause it’s bonus to go lower in correlation with my troop strength and troops remaining #, is there any truth to that?
% boosts apply to your current stats. So, % boosts gradually become smaller as your troop or spy bar depletes - because your base attack stats are supplied by the units themselves, not the actual buildings. Defense % boosts also apply to tower stats, so even when zt/zs, you'll still have the extra % boost applied to the static stats from those towers. Straight numbers (static boosts) essentially function like towers (although they are unaffected by % boosts, I believe). They don't change, and are always directly added onto your base stats.