I will refer to smogon singles since according to the vr of vgc, battle stadium singles and doubles ou, Slowbro isn't even ranked
In singles however, the reason it is good is, as the comments said, teleport and future sight. In gen seven, it was good but Toxapex was simply too good to use over it but Bro does have its niche in its ability to hard counter Mega Lopponny and depending on the coverage, Mega Medicham. In gen eight however, it has a much more difficult time since it horribly loses to Dragapult, Kartana and Rillaboom. It does have the benefit of completely hard countering Urshifu and slowing down Garchomp. However, even that is far fetched since Urshifu can simply you turn out while Slowbro cannot reliably stop Garchomp at all
Outside of these matchups, the biggest reason is teleport and future sight. Teleport is now a switch move with negative priority which means that the incoming teammate is almost guaranteed to not take damage while fs just allows for its teammates to have an additional damage against foes that would normally wall them. The damage taken by Slowbro when teleporting can be offset to an extent thanks to regenerator. For example, clicking fs and teleporting into a Weavile and now, Weave can suddenly beat a mon that would normally wall it in the form of Toxapex and Buzzwall. Weavile isn't the only benefactor as literally every Pokemon in the tier would love large damage inflicted upon their checks
After the Magearna ban, Slowbro began to fall off heavily because of its bad matchup. At the time, Bisharp, Weavile, Tapu Lele, Kyurem, Rillaboom and Kartana all ruled the tier. This led to regular Slowking taking over its job as a future sight teleporter but soon enough, Slowking started to fall off as well since while Bisharp did fall off, Weavile became top tier and Slowking horribly dies to Dragapult. However, in recent times, there are some experiments with Slowbro using colbur berry with body press to kill Weavile and ice beam to better handle Garchomp. It still dies to Lele, Kartana and Kyurem. It's not perfect but it's something. I haven't used this myself but there are talks going on about it in the smogon forums
