I will explain it in full grain terms. The grain to water ratio is needed to ensure the enzymes have the conditions to convert the starches into sugar. Too thick (not enough water), or too thin (too much water) of a mash does not create an environment that the enzymes prefer to do their job. (It gets really complex, thicker mashes are better for some enzymes, while thinner are better for others). Ratios and water amounts are given in recipes so you know how much water is needed for you to brew. Sounds obvious, but you dont want to waste time and energy heating more water than you need, or worse.... not heat enough. If you follow the recommended ratio of water to grain for the mash, and sparge it with the recommended amount of sparge water, you "should" end up with the recommended preboil volume. All this depends on your equipment (efficiencies, losses, preferences etc.) Partial mashes are more about getting the flavor from the grains than actual sugars from starch conversion (although that is a purpose as well).