Many things in brewing are a trade-off. For example, you can collect more wort from your grain bed to get more sugars, but then you have to boil longer to evaporate the excess water.
Another familiar trade-off is seen in milling. Generally, the finer the grist, the smaller the pieces of endosperm. Smaller endosperm pieces result in a higher extract efficiency. However, finer grinds also tend to cause problems when lautering because the pieces of husk are too small to form an efficient filter bed. As such, there is an optimal degree of fineness to a grist—fine enough to achieve a reasonable extract efficiency but not so fine that the brewer has trouble lautering.
Malt conditioning, or conditioned milling, is a way to cheat that trade-off. The technique allows the brewer to mill the endosperm of the malt more finely, yet still produce adequately sized pieces of husk for a filter bed.