Adolf Hitler had his own very personal reasons to radically increase the attendance of DAP meetings and its membership. Financial reasons.
Although, unlike most (if not practically all) his army companions he was still employed by the Army (which provided him with modest, but regular income), he knew that he will be eventually discharged – and rather sooner than later (he was discharged a month after the Party program was announced – on March 31st, 1920).
Which meant that he would need a job – or some other kind of a steady income sufficient to maintain the same lifestyle (and ideally to improve it). But he did not want a job – he wanted to be a full-time politician.
In other words, he wanted to work for NSDAP full-time. However, “working” was not an appropriate term as the party at that time was so short of money that it simply could not afford full-time employees. Thus, all party leaders (let alone members) had to have outside jobs simply to feed and clothe themselves.
Consequently, after he left the Army, Adolf Hitler became essentially an “entrepreneur politician” who made money for the party with his public speaking activities and took his modest cut.
Later he claimed that he received not a pfennig from the Party but made his living purely from the fees he received for speeches to other audiences he occasionally made.
However, in reality it was not the case. Although Hitler did occasionally speak to other organizations (other than DAP/NSDAP that is), the fees that he received were simply not sufficient to “keep his body and soul together” – let alone support even a modest lifestyle.
Hence, he needed to make money for the party on a regular basis – the more, the better. And there were only four ways to do it: (1) membership dues, (2) collections at meetings; (3) sale of publications – brochures, magazines and a newspapers and (4) donations from wealthy sponsors.