Well, I mean, yeah, it has more syllables hence it's more impressive sounding but lets get real. The reason we want this salt is that when you add it to water the components disassociate so that PO4 (Phosphate) can be freely absorbed. We really don't care too much about the Potassium (K+) because the plants are getting plenty of Potassium from Potassium Nitrate (KNO3) or from whatever primary Potassium source we should be using.
Monopotassium Phosphate (also referred to as Monobasic Potassium Phosphate)) refers to the fact that as a solid, this salt has PO4 ion magnetically bound to a SINGLE Potassium ion plus two Hydrogen ions. Schematically, the structure looks like this:
Look carefully at the image. Can you see that at the center is the Phosphorous bonded to four Oxygen atoms? That's what the term PO4 means. As a group, PO4 has a powerful electromagnetic attraction. Look at the three upper Oxygen atoms. While attached as a group each of those Oxygens have a negative charge, so the Oxygen on the left attracts the positively charged Potassium (K+), while the uppermost and right side Oxygens each attract and hold one positively charged Hydrogen. That's why the formula is written KH2PO4 = 1 Potassium (K+), 2 Hydrogens (H+), 1 Phosphorous (P) and 4 Oxygens. These atoms attract each other in exactly the same way as your refrigerator magnets attract each other, but the Oxygen bond to the single Phosphorous is much stronger than the group attraction to the H+ or the K+.
Water is a unique molecule because it is one of the very few liquids whose molecules are also strongly charged. It's called a Dipole because of the vast difference in electromagnetic charge at different parts of the molecule. Here are a couple of water molecules:
The red is the single Oxygen which is very much larger than the two Hydrogen ions, so the Hydrogen ions squirt around the until they form this Mickey Mouse head shape, exposing the negatively charged electrons on the backside of the Oxygen, while the ears have a positive charge. This makes water a powerful magnet with a North Pole(+) AND a South Pole(-). So water molecules hang around each other because the negative side of one is attracted to positive side of another.
This is the reason for capillary action, where the water can creep upwards on a dry rag if part of the rag is sitting in water. This is the reason water beads on a waxed car. This is the reason for a meniscus waterline in a beaker or test tube. Have you ever wondered how on Earth a tree that is ten stories tall can get water from the ground to the upper leaves at the canopy? The magnetic pull of water allows the one water molecule to pull it's neighbor up magnetically against gravity, and that neighbor pulls on the next neighbor below it. Only when the weight of the water column exceeds the magnetic strength of the magnetic bond does the column snap, and that limit of strength is what limits the maximum height obtainable by terrestrial plants.
So when the Phosphate salt is in water, the magnetic pull of the water molecules is strong enough, that the negatively charged Oxygen side rips away the K+ bond and the two H+ bonds, but the water is not strong enough to disassemble the PO4, however, the H2O dipoles move in, surround and isolate the PO4 from the rest. They also surround and isolate the H+ and the K+. This is why salts dissolve in water, and the strength of the magnetic bonds they have with their group components determines whether or not they dissolve completely or only partially.
Here is another more familiar example. This is common table salt made up of Sodium and Chloride ions. This is what it looks like when it's sitting on your table. look very carefully at the + or - charge of the Sodium (Na+) and Chloride (Cl-) and look at how the water molecules behave, turning one side to the Na+ and the other side to the Cl-;
But when moistened on your tongue or in your soup, this is what happens:
+
So guess what happens to Dipotassium Phosphate (AKA Dibasic, Orthophosphate). It has two Potassium ions and only one Hydrogen ion and looks like this:
Exactly the same thing will happen as in the Monobasic version. The water's electrommagnetic pull will rip the two K+ and single H+ and will surround and isolate the PO4 in exactly the same way. The only difference is that K+ is heavier than H+, so for a given weight or volume, there will be fewer PO4 and a little bit more K+ compared to Monobasic, but the difference is not enough to worry about because you can add a little bit more.
There is even a Tripotassium which does exactly the same thing when exposed to water. So really, it's pointless to worry about which version you have. For our intent and purpose, they are the same.
Cheers,
