According to Bernardo Ruiz in this article, the basic difference between a printed text and an online text is that while the first is static, the second flows, and is constantly subject to adjustments depending on the different types of monitors, slides, and browsers used. As this other article explains, “browsers aren't properly designed to justify texts”. The format finally adopted by the contents is therefore unpredictable, and that is why justifying a text does not make any sense.

The subject of readability is the second consideration. Several studies have proven that justified texts make reading difficult for those who struggle to concentrate on the words, or those with a visual impairment.

In summary, it's surely true that justifying texts is better left for printed media.