Due to extension of decay under the gumline, it may be necessary to
remove gum tissue (and sometimes some bone as well) in order to fully
expose the area.  If the tooth is re-crowned without a crown
lengthening, visibility is poor for making sure all decay is gone, and
the result will likely be a crown with the margin (edge) too far under
the gum, which will create a permanent gum problem in the area.
    The tooth must be evaluated for periodontal strength as well.  If
removing the necessary gum and bone tissue to make a good crown will
render the tooth loose, it may not be worthwhile.  Your dentist should
be able to advise you.

Steve

We agree Steve, a crown lengthening
may be necessary before a new crown is fabricated.

Some terminology.

Removing gingiva or gum tissue - > gingivectomy.

Removing bone or alveolus - - -> crown lengthening

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Joel34