SIL certificate or prior-use

The SIL certificate is of no value. It is the Report that provides the basis for the SIL certificate that is the thing of value.

As to why manufacturers pay money to get a SIL certificate, one possibility is that they use it to increase sales.

What report you mean?can you show me example (link,or upload)

I think PWSlack answered the SIL Certificate question.

The issue with prior use, or proven performance, is that in order to be a SIL rated system, there is a requirement for maintenance and testing intervals, and records to be maintained. These records will indicate the frequency of replacement or the reliability of the components, which will give a "proven in use" history. The calculations for the computation of the SIL level treat all instruments (in general) the same, such as all pressure transmitters have equal PFD's, and all Temperature transmitters have the same PFD's. SIL level is changed by adding redundancy, or changing the frequency of testing, selecting different valves, and a few other things.

Logic solvers are classified as triple redundant, double redundant, trip amps, relays, dedicated safety rated PLC, and so on...there are only a few types to choose from, and each brand is treated the same since they have a common PFD range as far as the calculations go.

Selecting field instruments and logic solvers for a SIS depends on whether it is an existing system or a new system. In either case, a LOPA, Layer of Protection Analysis is performed to determine what SIL level is required based on a company's risk tolerance. After the SIL level required is determined, the existing system is analyzed to determine the SIL level and any changes required to raise or lower it are designed. A new system is simply designed to that SIL level. Selection of field instruments may be from a company's current inventory or it may be necessary to specify an instrument to meet the new design conditions.