It secures which beat the chords are on. See attached as an example.
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Please compare the chords after import (mainly related to jazz chords) to make sure they are the same or equivalent chord symbols. There is some variability in music XML files in the way they are prepared (saved), then interpreted (displayed) after import, depending on each app.
For instance Dø could display as D or Dm7b5 instead; or F7#11 as F depending on the chord symbols available (or accepted).

(This is quite common even between two notation programs, where various elements, e.g. note placements, tuplets, spanning slurs are not interpreted or displayed as in the other program.)