Check the mod priority guide:
https://docs.google.com/spreadsheets/d/1ty239kyJbBs1sLhTo1JvSWuN0mIJsK1nwVdI6OLLtXo/view?rm=minimalDisclaimer: Note that AS Val is only useful for Night, as her damage multiplier is pitiful at Day.
With her MOD, AS Val rises from being one of the weakest base ARs to being a decent Night AR or surehit burst specialist, assuming she has the right support.
What makes AS Val MOD so much better than her base form is Faith, her Skill 2. Whenever AS Val receives a Damage buff from any source (Fairy Skill/Talent, HG buff, or even her own Skill 1), she gains surehit for 3 seconds. When combined with certain support, she can completely forgo using a PEQ at night in favour of a VFL, though this may not always be ideal, depending on the enemy and echelon’s needs. The extra stats and upgraded Skill 1 from her MOD1 also provide a smaller, but still appreciated boost.
AS Val also enjoys a very nice SPEQ-Set, increasing her surehit duration to 5 seconds, and granting her a 50% chance to gain 10 AP with every crit, capped at 200 AP. This makes her a lot more flexible as she can largely maintain her S2 on her own, and it helps when dealing with armoured enemies, which ARs typically struggle with.
All in all, AS Val is a decent Night Map AR, especially with her SPEQ-Set. While there is little reason to invest in a Night AR instead of other strong ARs who also work at Day, AS Val is technically a decent option who also comes with a potential niche when surehit is needed. Unfortunately though, one of her SPEQs is limited and cannot be obtained if you missed the event where it was sold, meaning that her SPEQ-Set is entirely inaccessible to some people.