The obvious benefit of catweaving (i.e., using Feral Affinity to deal damage as a resto druid) is that you can put out substantially more damage than you can with either
Affinité du gardien or
Affinité avec l’équilibre. If a fight doesn't require much healing or your group is very good at avoiding damage, you can spend your time adding meaningful DPS to a fight instead of sitting on your thumb.
There are sacrifices to be made, however. The first is that you have to take
Affinité farouche over
Affinité du gardien (
Affinité avec l’équilibre is mostly worthless for M+).
Affinité du gardien gives you a 6% reduction in damage taken and
Régénération frénétique which is a great tool for healing damage done to you. There are certain dungeons where I feel
Affinité du gardien is required like
Fourré Sombrecœur (Shade of Xavius) or
Salles des Valeureux (
Hyrja). Giving up
Affinité du gardien can be very scary, and a lot of healers won't want to give up that security blanket. There's nothing wrong with that approach, either. If you don't feel like you'd survive without
Affinité du gardien, I certainly wouldn't recommend giving it up.
Another sacrifice is your gearing choices. In order to unlock your full DPS potential as a healer, you'll have to wear damage-oriented legendaries. While catweaving, I'm typically wearing
Chevalière chatoyante (Feral legendary) and
La Dame et l'Enfant (Balance legendary). Neither of these will contribute a lick to my healing, and LatC will take up a tier slot so that I can't run my usual 4pc t21 and 2pc t19 that I like to heal with. I also run double DPS trinkets like
Racine noueuse de Chênecœur or
Balise de terminus. Trinkets are a powerful source of healing, so giving up those slots for more damage will impact your healing throughput noticeably.
Your stats are also an issue. The best resto druid healing secondaries are haste and mastery, but the best DPS secondaries are crit and versatility. When I'm going all-out on DPS, I try to wear as close to zero mastery as possible. This stat allocation lets me deal a lot of damage but also guarantees that if unexpected damage comes out mid-fight I will have a rough time handling it.
Gearing for catweaving is a spectrum of sorts -- you can keep some healing legendaries or trinkets, or keep your secondaries balanced instead of going full crit/vers. So you can choose how much you want to focus on DPS. But if you're really trying to impact a fight with your catweaving, you will be very weak as a healer.