At Contribution on the subject of cookie consent it has already been pointed out that the website statistics tool Matomo can also be operated without the use of cookies. This raises the question: Why are cookies used in Matomo at all and what are the consequences of deactivating them?

Why cookies?

Cookies are used in Matomo for several purposes. The first purpose is the Identification of returning visitors. If a Matomo cookie from a previous visit is found on the client computer when a website tracked with Matomo is called up, the comparison can be made with the existing inventory data. How often has the page already been accessed from this computer, when was the last time? How long did the individual visits last and - very important for web analysis - did calls to certain pages, such as advertised landing pages or product pages, only lead to the achievement of goals defined in Matomo, such as orders, contact requests or similar, on later visits?
Recognition is pseudonymous: a string of characters generated from random values stored in the cookie is sufficient as an ID for matching.
The second purpose is, somewhat paradoxically at first glance, the Storage of the refusal of Matomo tracking by using the opt-out function on the part of the user.
Finally, another purpose is the storage of the release for tracking, provided Matomo is implemented as an opt-in solution with an explicit request for consent from the user.
Details on the cookies set by Matomo can be found at [1].

What happens without cookies?

If cookies are not available to recognise users over several visits, Matomo instead creates a profile ("fingerprint") based on various properties of the calling device (device type, browser version, screen resolution, etc.) and stores it on the server. However, identification by means of the fingerprint is less accurate than with the use of cookies.
Consequently, the deactivation of cookies comes at the expense of the measurement accuracy of those characteristic values that are based on recognition. The value "Unique visitors" tends to become less accurate, while the pure number of visits, for example, is not affected. Also affected is the display of visitor profiles in the visitor log.
As a further consequence, it is no longer possible to assign target achievements/conversions to the original entry point over several visits. If a user comes to the website with a specific search term or, for example, via a specific link from an advertising campaign, but does not reach a goal until a later visit, this goal achievement is assigned to the entry point of this visit, for example as a "direct call", and not to the entry point via the advertising campaign.[2]
On the other hand, this allocation has limits even with cookies activated and only works if all visits are made with the same browser and no cookies are deleted or expire in the meantime.

Get rid of cookies, this is how

It is very easy for those who only track one or a few sites and want to deactivate cookies for all of them: under "Settings/Privacy/Data anonymisation", set the checkmark "Force tracking without cookies", then the script code provided by the Matomo server will automatically be adjusted accordingly, which will have an effect on all measured sites.[3]
If you want to use cookie-free and cookie-supported tracking in parallel within a Matomo installation, add the statement "_paq.push(['disableCookies']);" in front of "_paq.push(['trackPageView']);" in the tracking code for the cookie-free sites.

The finished tracking code together with the corresponding supplement can be conveniently generated automatically under "Settings/Websites/Tracking Code". To do this, click on "Show" under "Advanced", then set the check mark for "Disable all tracking cookies", copy the code then displayed in the text field and paste it into the target website.

[1] https://matomo.org/faq/general/faq_146/
[2] https://matomo.org/faq/general/faq_156/
[3] https://matomo.org/faq/general/faq_157/

Continuing:
https://matomo.org/faq/new-to-piwik/how-do-i-use-matomo-analytics-without-consent-or-cookie-banner/
https://matomo.org/blog/2018/04/how-to-not-process-any-personal-data-with-matomo-and-what-it-means-for-you/