There is an obvious answer to this question: large IT systems bring invaluable business benefits. We all know about the incredible infrastructures of Amazon, Microsoft and Google; X5 Group also has similar services. But we will talk about how a large system can be useful for a programmer,
Any specialist who improves his skills sooner or later notices that he has received all the knowledge and skills that are necessary for the current work. This often happens when you get stuck on a small project for a long time. This "glass ceiling" indicates that the business tasks that an employee can receive in small companies have become too simple for him and no longer inspire him to conquer new heights.
But there is no need to despair: there will definitely be points of growth, the main thing is to look for an application for your talent, and if one day an offer comes from a large company, do not miss this chance.
When you start working with large systems, you will realize that even for simple tasks, the requirements here are completely different. Much of what is familiar to you, senior developers will call unacceptable and intelligibly, with examples, explain why this is so.