Still can be used for specific details

Prior to being nerfed, dropping a dragon egg in a lazy chunk would break bedrock. This was the simplest way of doing so and often the reason for mass duplication the egg as it made clearing the top four layers of bedrock inside a spawnproofed perimeter a reasonable (if still quite daunting) project.
[​Or the 0-layer as well, although this would lose an egg for each location cleared, but allowed the most efficient spawning in farms.]

trophy

What ScotsMiser said.

Decor/ aesthetic usage has been previously traeted as useful for a variety of items.

As a practical matter, glowing item frames and glowing signs can be useful for pathfinding through dark areas (assuming one doesn't just crank up the gamma) near mob farms without needing to add light (which can decrease spawnng efficiency).
[A fairly esoteric use (and one probably only likely to see the light of day 😎 ) on servers that block the gamma adjustment dodge.]

the fancy frame would be a use tho

Snowgolems are also useful in certain mob farms.

Prior to being nerfed, the dragon egg was wonderful for bedrock breaking.

Ender egg:Trophy
Dragon's breath: tipped arrows
Snow golems: snowballs
Fish: bones for bonemeal and stuff

Randomizers

Ambience

Kill one for possible loot in multiplayer

Get a jukebox and use them

They can zombify villagers for trading discounts

pretty sure the knowledge book is used to unlock recipes the player wouldn't otherwise have access to, such as in adventure maps and the like

put it as a trophy in an item frame for an illegal item

glow ink sac

drops glow ink, which can be used to make signs vastly more readable. also, adds a really cool atmosphere to the underwater caves that makes them feel very different to regular caves and regular water.

teleports the player to a random safe place when eaten

Other than pretty designs, these can be useful in redstone when one wants a pushable (but non-pullable) block that does not adhere to slime/honey

decor

Player heads aren;t a vanilla item (and mostly get used for posable armor stand art and bragging rights)
Mob heads are wearable — although the benefit is so small and the heads so difficult to obtain one wonders if anyone actually does this (other than as a stunt)…
q.v. https://minecraft.wiki/w/Head#Wearing

wear them to look funny or mention someone

Lillypads can be very useful when starting mid ocean builds if one does not wish to pillar up (blocks or kelp) from the seafloor.

Mossy cobbblestone is the cheapest stone block that is immune to silverfish infestation.

Snow blocks are needed for snow golems (highly usefulin certain farms)

Prior to being able to farm podzol, mycelium was the only renewable block on which mushrooms could be farmed at high light levels.

Dead bush: yeah, that one's pretty useless other than for decor and a nonrenewable source of sticks if one gets a desert spawn…

mossy cobblestone is really helpful for transitioning from green to grey, which is something that really helps when building with stone in a grassy area.

mycelium is one of two blocks that mushrooms can be placed on regardless of light level.

healing: want to heal ur friend and u have nothing else
lily pad: parkour
Mossy cobble: decor
Snow: snowballs
Mycelium: decor
Dead bush: sticks

Bubble columns are great not only for smaller & faster item elevators (c.f. the pre 1.13 designs), but also as a replacement for the "bubblevators" in many situations (although bubblevators are still useful for some sorting functions ).

oh lol