The answer tends to depend on what you are optimising for. Some select us based on our extensive experience handling ecommerce businesses for the peace of mind. Some select us in order to save them time based on not needing to explain how their business/platform/dropshipping/etc. works, our efficient processes for handling ecommerce or the fact we get your Sales data for you. And some choose us because they have been paying higher fees to a non-specialised accountant who is charging hourly but being inefficient in the delivery.
It tends to come down to a combination of having confidence that your accounts are being done right, time savings, and value for money.