The fascination is two-fold: firstly, his astounding arrogance that we should care about his domestic trivia and narcissistic scribblings. Secondly, we can all sympathize with (and wince at!) this ridiculous slave to convention.
Above all, Mr. Pooter's life is funny! His constant battles with tradesmen, his pathetic pride and banal wit, his clashes with his carefree son, his absurd social crises and petty dilemmas: all are part of Mr. Pooter's life as a worried, proud, and anxious Nobody. Listeners are certain to learn why Hilaire Belloc asserted that Pooter was "an immortal achievement."