While the primary use of hashtags is to help people find content based on categories, custom hashtags can be a great way to get people interested in your brand and encourage them to share your content. Moreover, you can use the same hashtag across multiple social media platforms.
Take this custom hashtag from Marvel, for example. In 2015, Marvel created the hashtag #MarvelHalloween. Hundreds of fans used the hashtag alongside photos of their own Marvel halloween costumes. Not only was this free advertising for Marvel, but it was a great way for them to foster consumer engagement with their brand.