Can someone clarify why the legato articulation is needed in notation? Staccato 'shortens' the duration but surely the default is legato? How can a note be played more legato?
Legato as an articulation is needed, because it is not the default "goes without saying" articulation, at least not universally for all instruments and styles. And legato slurs are needed to show which notes should be played as a single phrase, which couldn't be done by writing a single word "legato" somewhere.
If you listen to players playing notes with "default" articulation i.e. without any special articulation instructions or markings, or look at the waveforms, you'll notice that each note is - or can be - cut in advance of the next one. No specific effort is made to restrain the attack of each note, and it's OK if the listener has no trouble distinguishing separate notes. Legato means trying to glue or bind the notes together so that sound isn't cut, and on piano or organ the notes can even overlap. Staccato means making the separation of the notes pronounced, usually by shortening the notes, but it can also mean making the attacks sharper or otherwise more pronounced.
How different articulations are played isn't universally precisely defined, and depends on the instrument and player and can be adjusted according to what the players and/or conductor or producer see fit. And legato vs staccato etc. is not only about note length. Legato means "in a bound way". If they had wanted to say "with longer notes", there would have been separate words for that in Italian. If you look at the attacks of notes played staccato, on average they are not the same as attacks of notes played legato, except on instruments where you have no choice. So, legato is not only about how the previous note ends, it's about how the next note starts as well. On a pipe organ, there aren't that many different possible ways to start a note, but other instruments have different capabilities. The technique used to play "legato" can vary, and for example on the guitar there are many different left-hand and right-hand techniques that can be used for legato, if there are no guitar-specific instructions.
Legato slurs are a related thing, and they are used to mark phrases, inside which notes are to be played legato. But a phrase isn't strictly only about how connected the notes are. Just like a phrase can mean many different things in speech, a musical phrase can have other dimensions besides just separation of notes. And this depends on instrument and interpretation.
There are several questions related to what the default playing style should be, like these:
Probably more to do with phrasing. From beginning to end of a legato phrase, all the notes will be joined 'in one breath', but at some point, that phrase will end, and a new one start. Those end and start notes won't be legato. There are several stages between staccato and legato, so it's rarely one or the other.
On many instruments, connected notes do not have to be attacked. On those instruments, a slur means to only attack the first note in the group. For example, on the flute, one would only tongue the first note in a slur.
With instruments like piano, there are several possibilities:
- Phrasing. Legato not only means connected notes, but also notes diminishing in volume that make the phrase smoother.
- The character of the piece may imply a more detached playing style, in which case the slur is needed.
Additionally, the slur is sometimes used in baroque music with notes inegales for other purposes. https://en.wikipedia.org/wiki/Notes_in%C3%A9gales