NASA's Perseverance Mars Rover

Shortest answer: quartz has to be separated from other rocks/minerals. Water action is one of the easiest ways to manage that. In addition, opal/chalcedony is actually quartz with water directly attached on a molecular level, so that's a direct discovery.

Medium answer: Igneous ("volcanic") rock already contains the silicon and oxygen that quartz is made of, but they're usually bonded with other elements, not just each other. In other words, they don't exist as "free quartz" - meaning independent grains that are made of pure SiO2. As @athairmor alluded to, free quartz can form directly from magma when it solidifies and forms igneous rock. However, that is what you would expect from particular kinds of volcanic rock, which are absent or rare on Mars (e.g. granite). The igneous rock around Jezero Crater is not the type to contain "free quartz". If the regional geology hasn't served up any free quartz grains directly, you can still separate out the silicon and oxygen by breaking down the larger, more complicated minerals they're attached to, but that would take a significant amount of chemical breakdown - i.e. significant amounts of water. This process is quite common on Earth, of course, where it yields up "white sand" on beaches - which is simply rounded grains of quartz.

Longest reply: I should probably just read the EPSL paper, and I'd be happy to summarize it here if people are interested.

Quartz can form through a couple of ways. It can form from magma as it cools or crystals will form from silicon dioxide dissolved in water as the temperature or pressure drops.

The scans of rocks on Mars show the pattern of hydrothermal forms of quartz. This might happen when a meteor hits causing an increase in temperature and pressure near the edges of the impact crater. This might dissolve silicon dioxide in any water present then, when it cools, quartz crystals form.

I don’t know what they mean by “water circulation” but it does point to water being present.

These rocks therefore bear witness to very ancient water circulation on Mars, and are interesting from an exobiological point of view. Siliceous rocks, and opal in particular, have a remarkable capacity to preserve traces of life, whether morphological or molecular. If Perseverance succeeds in sampling this type of rock, it will be a prime target for the search for signatures of life once brought back to Earth.