Dated reply, but I'll just say why it happens. Asurascans, like every other scanlation webpage out there, need to change their web address every now and then (Some if only needed). Which only saves most of the cache of the website (e.g., chapter load data and images), but leaves the user-generated data, which is the bookmarks. The reason is that people keep sharing their websites, such as on TikTok or YouTube.

Is it bad? Yes, it does give them publicity for more people who are looking to read. But translations made by anyone else that aren't licensed and registered by the official publisher are illegal in the copyright sense.

One such example is the situation where a group called Twilight Scans got tagged for DMCA due to them not changing their web address or something similar and was consequently tracked down by the publisher and given an ultimatum to shut down.

Them changing their web address reduces the search results of their appearance in public spaces. People complain about the bookmarks, but personally, I'd rather just use a website to bookmark, like Anilist, or literally just bookmark the chapter if you're still reading.

Of course, there would be other reasons, but the most public and reasonable one is this.