It would affect an aircraft's actual altitude above obstacles. If flying from high pressure to low, your actual altitude is decreasing despite your altimeter reading the same. If not careful, you could inadvertently fly into an obstacle which would have been well below your altitude when you were in the higher pressure zone. Every hPa is a difference of around 27 feet, so that difference of 26 hPa means you'd be flying 700 ft lower in 999 hPa than you would in 1025 hPa.
The saying goes "Flying from high to low, watch out below".