Like most physical media it is virtually an [anachronism] in the [21st century] when [word processor]s and graphics applications and their host devices are so pervasive and well-supported. Yet the old [medium] has many qualities - summed up in one word, [versatile|versatility] - which make it more suitable or convenient.

cost: cheaper than a [moleskine] or similar brand of bound blank book, far cheaper than a [iPad|tablet]
availability: most stores that sell stationery carry this type of notebook, but university bookstores have the lowest prices and the widest variety - blank unruled, wide-ruled, narrow-ruled, grid-ruled
durability: the pages are stitched with heavy thread and the spine is wrapped with a kind of fabric material; the cover is stiff enough to bear down on for writing in the field yet flexible enough to fold in half if needed and still maintain integrity; it won't break if you drop it on the pavement or leave it out in the rain (things that would break a computer)
flexibility: [blank book with intent to collage|paste pictures or draw on the cover to suit your taste; staple or paste extra pages, pictures, graphs, field samples to the existing sewn-in pages]
longevity: barring [hurricane|extreme] [volcano|calamities], your comp book should last for centuries or even millenia; will never require electricity or any device other than your eyes to read it

Any of these potential benefits may be rendered [moot|irrelevant] if you ever want to share your work, but if quality isn't an issue, [scanner|scan] it or take snapshots with your [Instagram|phone]. But the kinds of writing best suited to comp books are the kinds you don't want to share:

[free writing]
writing practice
drawing practice
personal [journal] (nobody wants to read your [blog] anyway. I'm sorry.)
[burn book]
drug dealer's ledger
gambling ledger
real accounting books
[Se7en|murder diaries]

Other good uses for comp books, things you might want to share:

lab notebook (often required for chemistry and biology courses)
hiker's journal
[geocaching] cache [geocache|log]
fake accounting books

The world is undeniably moving away from physical media toward [digital media]. But consider the [instant gratification|immediacy] and [permanence] of a comp book (or any physical media) for you next project; compare the cost to that of making a [hard copy] of a digitial work. [Printer ink] is notoriously expensive. It is much cheaper to go in the opposite direction, to make a digital copy of your physical work.

composition notebook (thing)