If you've ever owned an older home, you've probably had to replace either the roof or the siding, if not both. The general thought process to the layman is to keep the repairs as cheap as possible while achieving the desired results, and for many people that's what has to be done. However, if exterior repairs are required, it's usually a good idea to at least consider the idea of replacing the roof and siding at the same time.
Weather Damage, Wear and Tear
The first reason to consider replacing both the roof and siding is they have likely been on the house for the same amount of time therefore they have received the same amount of wear and tear while being subjected to the same weather and potential damage. If the winds of a heavy thunderstorm have damaged the roof, hail has likely damaged the siding. Likewise, if the siding has become too old to properly clean, possibly missing pieces here and there and needs replacement, the roof is probably compromised as well.
Matching Styles and Colors
Another reason to replace both at the same time is if you plan to change the exterior style or color scheme. You may change one and plan to change the other later, but then you have a house which doesn't match for those years. A further consideration is if you plan to pay for the project with a home equity or credit based loan. You may be able to arrange a better deal on the larger loan to pay for both the roof and siding at once than using two separate loans to pay for them individually. Even worse, something may change over the next few years which prevents you from getting the second loan, or prevents you from getting it at a reasonable rate, and so the project never gets finished.
Investment Homes and Flipping Houses
A third instance in which it makes sense to replace the roof and siding is when flipping an investment home. Replacing both is going to add value to the home and just as importantly, add curb appeal by making the house look new. It's a great selling point when negotiating the counter offers potential buyers give to point out both the roof and siding are brand new. Further, when they see the effort you put into the exterior the buyer realizes you've done a great job with the interior as well, assuring the house meets their standards for a home.