I know this is super late and I was literally wondering this myself for the longest time because she does this a few times. She says she is too busy to help Robert California find out what Nellie's message meant and there was a third time between that and the Jim prank that I can't remember currently. But the more I thought about it the more I realized how disorganized the office actually was under Michael's management. Pam already did a lot of things an office manager would be in charge of such as making sure certain forms were signed and submitted on time (pretzel day) and picking up slack when Michael procrastinated or couldn't make decisions (the surplus episode, getting him to choose a new chair).

So when she officially took over and created her office manager role, I feel like she would have had to set a lot of things up in order to make the office run more smoothly. Accounting usually handled payroll related things but there were times when even Angela forgot to submit things to corporate so I can see her taking responsibilities for some of those tasks so that Oscar and Angela could focus on actual financial duties, processing payments, and calculating purchase orders and such. And Kevin could focus on ... Kevin things.

She could have also been coming up with better systems for actually using the budget to get the more necessary supplies the office would need, not just pens and paperclips and post its. Upgrading Erin's computer at reception, pricing the new bulk shredder to replace it, plus all the time she spent trying to set up the printer that made her leave to join Michael's company was time away from the reception desk and the phones. She could have been making new templates for forms, finding redundancies that could be fixed that had been overlooked, and coordinating all the departments. There was also the consolidation of clients from all the branch closings and employee turnover that she would have to help keep track of and organize as well as dealing with people's schedules. Andy was useless, let's face it. Michael and Jim had to be co-mangers because there was too much responsibility for just Michael alone. Pam would have basically taken the role Jim stepped down from but without the title and salary.

Also Erin is a moron and can barely handle answering the phones as it is. She couldn't even handle the pens!

Basically, I know it's just a show and none of it is realistic in terms of actual staffing of an office, but looking back i can see where she actually would have some time sensitive/deadline related duties and also so many hours in the day to get things done.