When a store needs a new feature, there are three routes. Start with the cheapest that fits.
1. An existing app
Best when the feature is common, such as reviews or subscriptions. Check speed impact, support quality and monthly cost.
2. A theme change
Best when the feature is about layout or content. A new section or a metafield-driven block often does the job.
3. Custom code or a custom app
Best when the feature is specific to your business, or when apps would add cost and weight. It costs more up front and needs maintenance.
A good rule: try the options in this order, and move down only when the one above cannot do the job.