Appeal to a wider audience
In general terms, direct marketing is more effective than online marketing, because it is more focussed, based as it is, on detailed audience analysis. However, not everyone in your catchment area likes looking at promotional material posted through the door.
Nothing to do with your campaign, but some customers absorb all their info – including marketing – through online channels. By using the same client information and channelling into a simultaneous online advertising campaign, you can reach more of your target audience.