In earlier days of tarot reading a significator was cognitively chosen by the reader to represent the querent. The choice of the significator did not add to the interpretation of the reading in any way. It may have been simply part of tradition. It may have been seen as a way for the reader, the querent, the cards and the Universe to connect.
Significators were chosen from the sixteen Court Cards based on age, gender and hair color.
Only a few modern tarotists continue this tradition today.
Many tarot spreads have a significator position. The card that randomly falls into this position describes who the querent is at the moment of the reading. This can be very helpful information in a reading.
Significators have an important place in tarot magick, too.