When a product is ordered using an order form, the order status is set to 'processing' by default. If you want new orders to default to 'completed', you have to set 'auto fulfil orders' to 'yes' on the product record.

The theory behind it is this: A user places an order using an order form. A pending order record is created, but no 'real' order record exists yet. The user then pays using a payment gateway, or sends you a cheque and you activate the pending order. Now a real order record exists, but unless the product ordered has 'auto fulfil orders' set to 'yes', the order status is now 'processing'. Then you actually provide whatever product or service it is that has been ordered (eg. you set up their hosting account). When that's done, you go back to the order record and manually mark the status as 'completed' (or 'dispatched', or whatever).

Setting 'auto fulfil' to 'yes' on the product record negates the need for manually going in and updating the order status. This is intended for use with downloadable products or user subscriptions where no action is required on the part of the administrator to complete the order, but if you're just lazy, you can use it regardless.