Once you’ve built your ad, you still need it to be “served” to your target audience via a publisher’s platform. Every publisher is different, with their own unique set of media specs and requirements for digital ads.
If you want your ads to be seen and perform efficiently, their creative files need to meet every criterion for the destination platform. Otherwise, you risk putting resources into ads that won’t be seen. Optimizing digital ad files requires an in-depth knowledge of publisher requirements and coding, something that a DIY platform can’t reliably provide.