Custom software is a fully tailored graphic Design that is made especially for your needs by a certified designer. It is different from off-the-shelf template which doesn't always meet the specific needs.
If your project idea is so unique that you can't fit it into any existing template, or if you want to make granular changes that are beyond changing the colour and fonts of a template, you may need to consider getting a Custom design for your project.
Custom Design is made for you and for your needs, resulting in a design that is more beautiful, tailored and user friendly. Usually, investing in a custom design once will mean that you wouldn't need to make any changes for a while.
Yes, you will always be part of the process and will be part of any important design decisions.
It's free, non-obligatory and you won't be signed up to any mailing lists.