A blender is treated as a shortcut for chopping very finely. It performs three additional actions at the same time, and those are usually what determines whether the result is good.
The blade shears rather than chops
The blade spins fast enough that food is not so much cut as torn apart by the difference in speed between the moving liquid and the material in it.
That shearing ruptures cell walls comprehensively, releasing contents that a knife would leave inside intact cells.
Some of those contents are enzymes and compounds kept separate inside the living plant, and once mixed they react with each other in ways that never happen on a chopping board.
Released enzymes change flavour within seconds
Certain vegetables and herbs contain enzymes that generate bitter or pungent compounds only when cells are broken and their contents combine.
Prolonged blending gives those reactions more time and more exposed surface, which is why an over-blended green sauce turns bitter while a briefly blended one does not.
Cold slows the reaction, which is the practical reason chilled ingredients and short pulses give cleaner-tasting results than a long run at full speed.
Friction heats the contents measurably
The energy driving the blade does not disappear. Most of it ends up as heat in the food, and a long blend can warm a mixture noticeably.
That heat drives off volatile aromatics, dulls fresh colours and can begin to soften delicate emulsions that were stable when cold.
It is also why frozen preparations are blended in short bursts, since continuous running melts the very thing being made.
Air gets folded in whether it is wanted or not
The vortex in a blender jug pulls air down from the surface and distributes it through the mixture as fine bubbles.
Incorporated air lightens colour, which is why a blended sauce often looks paler than its ingredients, and it accelerates oxidation of anything sensitive.
Filling the jug fuller reduces the vortex and therefore the aeration, which is a simple adjustment that changes the finished appearance considerably.
Particle size determines perceived texture
Smoothness is a function of how small the largest remaining particles are, not the average, so a mostly smooth puree with a few fragments reads as coarse.
Fibrous material resists shearing and tends to survive as those fragments, which is why straining after blending changes the texture more than blending longer would.
Very fine particles also release starch and pectin that thicken the mixture, so a puree can get thicker as it is blended without anything being added.