Hello Tim,

The Contacts are already used to track page visits, event bookings, registrations, product purchases and newsletter subscriptions, as briefly summarized in the Overview for Contact activities article.
Next articles branching from this guidepost describe configuration of activities logging, etc., including custom activities logging.

The real use depends on your aims and requirements.
An example use of activities can be scoring, which can be one part of larger Marketing automation process(es). Those processes together with use of Reporting features may bring "the truth" about contacts.
You can even use the collected data in Website optimalization and Content personalization, described in other chapters...
As for seaching in Contacts - you can use the filter/search controls in the listing, as shown at Contacts management page description.

Should you need any additional details, please feel free to ask.

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Thank you.

Regards,
Zdenek