Pilz Relay info

[EddyCurrent]

It's a much bigger picture. It starts with law and making equipment safe, such as the Machinery Directive.

Then you have to make an assessment of the required safety category based upon the criteria here, although in practice it's much more involved.
Standards for functional safety - Pilz - GB

Only then can you choose the appropriate safety equipment to achieve that level, basically 4 categories.
To achieve a safety level, relays such as the Pilz have to demonstrate that they will not fail within certain criteria and they have increasing levels of self checking, only then can they be certified as meeting the standard.

So to answer your question, the relays are designed to achieve a particular safety category to satisfy the requirement by law to make machinery safe.
Your PC, bob, standard relays, have not been certified to meet the standards required. In a domestic setting at home in your shed these safety requirements do not apply but because these relays can be found on ebay at good prices I don't see any reason not to use them, and they can simplify the control circuit too.