The quick answer is "because of best practices". The real and powerful answer is: because the percentage reduction of the ECC database achieved with archiving before the upgrade to S4HANA will be directly proportional to the savings achieved on the HANA infrastructure investment required for the conversion. It is measured in money. So, clearly, it is worth considering.
It is true that you need expertise that you may not have in your organization.
At Krokios.com we can help you, making an initial estimate (free of charge). This will lead to the design of a concrete and reasonable strategy to achieve the savings objective.