Use existing fields on custom activity for web leads

We are now only use API to push website leads to Marketo, and exploring the possibility to create custom activity for inquiries, downloads and registration etc.

After reading all the documents online, I'm still a bit confused about one thing. Do I need to create custom activity fields if they already exist on lead profile?

For example, when a download happens online, it will push over a series of values for lead email, download asset name, product category, etc., which all exist on the lead profile already. Do I still need to create them under the custom activity?

And of course, those fields will get overwritten every time, will creating custom activity fields help preserving the historic record values?

Thanks in advance!