if you want your watches to be at its best. This is based on the assumption that one only has one watch and it is one's everyday watch so it is fair to assume that there's a fair amount of wear and tear in the movement. A simple part like a winding crown for example is subject to a fair amount of tear and wear over the years. Personally, I think 7 years is a fair interval for me as I rotate my watches and some watches hardly get worn (with those, you need to give them a good wind at least once a month). That said, I have sent in watches at shorter intervals.
By the way, the recommended service interval info is based on conversations I have had with Lange watchmakers during my factory visit and at Lange events (fortunately we have quite a bunch of events over here in Singapore). I prefer talking to them than to watch sales people because they are more "honest".
Hope the above helps.
Best,
Ian