• to enhance general understanding, and therefore appreciation,  of music
• to aid the ability to „read“ music
• to help in learning to play an instrument or sing
• to pass exams
• to satisfy curiosity
• to get trivia quiz questions right
• to show off!

You may have other reasons!

* The Greeks and Romans both had non-graphical notations which used letters of their alphabets to symbolise notes. From this came our use of the letters A to G to represent notes which is still common in many countries.
o The letter names are sometimes called the „Boethian notation“ after Boethius, a Roman writer and statesman who lived in the 5th century. He was in the service of Emperor Theodoric, was accused of treason and executed in the year 524 A.D. He was the first to document the use of letters as names for notes.
o An alternative method of note naming was introduced about 1000 A.D. by the monk Guido d’Arezzo. This has survived up to today as tonic sol fa. The most important aspect of this development however, is that it used six of the notes which we use in the major scale today.
o France, Italy and other associated countries now tend to use the tonic sol fa names (based on C as Doh) as names of notes, rather than alphabetical letters, but this change has (I believe) only happened in the last two hundred years.

* Early systems of notation which used letters of the alphabet were the origin of some of the symbols used nowadays
o In early times, B flat was a different note, and a rounded, lower-case „B“ was used to represent it. From this comes our use of a b for a flat sign.
o A squarer, gothic, lower-case „B“ was used for B natural, and from this comes our natural sign:
o Our sharp sign comes from this gothic B with a line through it:

* Modern notation is much more precise than older notation.
o When I was a boy in an Anglican choir, we used a hymn book which had some hymn tunes in old plainsong notation. (I still have a copy in the cupboard in fact – the English Hymnal of 1933).
o This plainsong notation uses a four line stave instead of five, no time signatures or key signatures, and has some diamond-shaped notes.
o This notation, compared to modern notation, is quite imprecise in its specification of how the music should be performed.
o However, this was probably good enough for the style of music it was used for.
o This is also true of even older notations: it may seem sparse to us, but it was appropriate and sufficient for the type of music it notated.

* Graphic forms of notation are first known from the seventh century
o The earliest forms of graphical notation were probably just marks indicating approximate pitch to remind readers of a tune they had already learnt. These would have been used by strolling minstrels and monks in monasteries.
o This evolved in church music into plainsong
o Plainsong was at first very imprecise, without clefs or staves
o The modern system for notes was developed initially in the fourteenth century.

* The features of modern notation are there in order to notate music that we know today.
o Over the years, many experiments have added new signs, new methods and new complications.
o Those that proved useful for the music of the day have stuck
o Those that were complicated, cumbersome, or not useful have mostly been forgotten
o Unfortunately, some old music uses obsolete signs, and in some cases it is not even clear what they mean

* Modern notation developed in Europe and spread to the rest of the world.
o This makes music notation one of the most widely recognised international languages of all time