Voice notes recorded in pieces, interview answers saved one by one, or parts of a talk: eventually you want them as a single file you can send.
Clips from different phones and apps often use different settings, such as mono against stereo or a different sample rate. They are all brought to the settings of the first clip, so nothing plays too fast or too slow.
A second of silence between answers makes a recorded interview much easier to follow. For music or a talk that should flow, leave it at none.
All the clips are held in memory while they are joined. If the list is very long, join it in smaller groups and then join those results.
Yes. Any audio your browser can open can go in the same list.
Saving as WAV loses nothing. MP3 encodes the joined file once.
No set limit. The practical limit is your device's memory.