Provided you can keep them free from insects and away from rodents, biscuits being so dry was a plus. In days before preservatives biscuits, crispbreads, crackers, and so on were things that kept (and still do) keep for quite a long time.
Next time at a shop or supermarket examine date certain biscuits, crispbreads, crackers, cookies, etc.. were made, then look at their sell by date. There is often a considerable amount of time between two.
This also is where all those lovely biscuit, cookie or whatever tins came in handy. Long before plastics, vacuum sealing or whatever tins sealed out moisture which kept contents dry and crisp. Tins also kept out vermin which were far more frequent even in best of homes, and certainly shops were food was sold.
For a seafaring nation like Britain (or anyone else) biscuits became a staple of provisions for sailors and others on board. Again their being dry meant would last without going off or moldy. Hence the phrase sending someone to sea "without a biscuit".
Sea biscuit, ship’s biscuit, and or molar breakers, took dryness of biscuits to a whole other level. As name implies they were often rock hard and tasteless. Then you had fact these biscuits often were soon infested with insects and or their larvae. If issued as part of ship's rations you had attack by rodents to worry about as well.
Habit of dunking biscuits, crackers or whatever by military or anyone else came about due to often occurrence of insect or larval infestation. Food being expensive, and not always easily replaced dunking biscuit in liquid (such as hot coffee, tea, milk, etc...) caused insects and larva to float up and into liquid. You then scrapped off any that clung, and consumed biscuit. Sounds gross, but as mother would say "if you were starving you'd eat it...".