Implementing an ISMS means that all data will handled consistently and securely. It means processes are documented, trained, and repeated for consistency. This gives your customers piece of mind to know security is not just a statement but apart of the culture.
If you implement ISO 27001 to achieve the ISO certification or SOC 1, 2, or 3 an independent auditor validates the effectiveness of the controls within the ISMS. This proves to your potential customers that you can be trusted. It also shows maturity in your processes.