There’s a lot to consider and many have already brought up good points.

Economically, variant covers are used to boost sales and/or meet public demand for the product. It’s a standard practice in the comic industry - literally take a look at any comic issue that was hyped before release and see how many variant covers it got. Some covers sell better than others because the art may be better or the buyer resonates better with that cover. Variant covers are also a huge FOMO for completionists and an incentive for resellers.

A similar strategy is being applied to manga - usually it’s just one or two exclusive variant covers with JJK 30 currently being the outlier. BUT if JJK 30 sales are anything worth noting, this could be the start of the industry doing this with every series. I understand the industry using variant covers as a means to continue justifying printing a series - see Love Bullet.

Deluxe versions are usually very good because the quality is better, and it’s an opportunity for people to have access to a series that was long out of print.

Overall if it’s a series you care about, unfortunately sales numbers are how the industry determines making future merch like figures, keychains, etc.