Hi Carly,
This question probably would probably get more answers in the "products" section.
There are quite a few answers in the community on this, but some are having some errors, hence the fact that I will not point you to any one.
Bottom line is, except if you manage a portal in your web site and you need the login forms, most of the time, replacing these forms with Marketo ones is preferable. If you do not, you will absolutely need to integrate your CMS forms with Marketo and use for this the forms 2.0 API (so that form fill out events are logged in the Marketo activity logs).
But behind this principle, as always, the reality is more complex: if you have a (very) large number of existing forms in your CMS, replacing them all can be a cumbersome task. It has to be planned and executed thoroughly and if you do not have the resources, you will have to keep your CMS forms at least for a while.
To guide you more, what CMS are you using?
-Greg
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