Re: Question about old tournaments
I am certainly not the expert, but I believe that the answer to this question varies depending on the era. Post WW2, until the latter 50s, the Americans dominated the tennis landscape. The American tournaments (particularly the East Coast grass tournaments) and the British grasscourt season were the most important tournaments outside the Slams. Often many of the top Americans skipped the European clay events. Some of the California hardcourt events often held great significance. But if you look at the ranking lists for those decades you'll see that they were linked heavily to Wimbledon and the US championships and the events that were held just before and after those events. The French and other European clay events were given weight based on whether the top Americans played.
By the late 50s, tennis began to take a more international flavor. Players from Australia, continental Europe, South Africa, and Latin America raised their levels of play, thus leading to the significance of worldwide events. By the late 50s/early 60s, the British and US grasscourt events still were significant, but the South African, Italian, and German championships were as or more important. Sometimes the winter/spring Caribbean circuit had a powerful entry list. The Pacific Southwest event held right after the US often had one of the strongest fields of the year.
The landscape again changed significantly in the late 60s with the advent of open tennis.