You could create all your pricing pages, for your free trial, paid trial, monthly and yearly plans, quarterly, semi-annual, multi-years offers. Plus any promotions that's going on. You could build your own checkout and self-service experiences for cancellations, renewals, upgrades, downgrades, change payment, change address, pay invoices, change the number of seats or licenses etc... Then integrate all the subscription changes with your billing, payment gateways, and CRM.

That's just the basics. As you grow, you'll need to handle different currencies, duplicate UI for different languages, different tax rules and payment options, more devices. You'll start creating different UIs for different subscription products and customer segments. You'll get fraudsters, PCI compliance issues, and subscribers abusing your special offers, so you will need to build logic to block off certain type of purchases or changes. You'll need to build your own cart logic, your own promo code and handle complex bundling scenarios. You'll have to integrate with your CRM, so that your sales and customer service people to be able to assist subscribers at any point. You'll launch new products, new promotions, new pricing, new branding, and need to update it all over again.

Your developers will be building and maintaining this instead of working on your product - forever.