I guess it was common practice in that era. Charlton played at least one such game per season until 1960. Newspapers in mid-August were full of reports of teams playing in-house trial games and inviting fans to watch. I expect supporters were charged to get in as well, or crowds would probably have been bigger. By the way... I missed a friendly game Charlton played at Griffin Park on 20th February 1937. Here is a link to the post it has now been added to... All Charlton first team matches from 1905