Some detectorists go their entire lives hunting with the same coil, but many others will invest in an additional search coil that helps their hunting style in one way or another.
A search coil that you invest in has great potential to improve your metal detectors performance in a big way. Often metal detectorists will look for a DD search coil that offers you greater depth or higher sensitivity, and this greatly improves the detectors ability to find small targets at much deeper depths. This is very important as many tiny objects such as gold nuggets and small relics can be too deep that a normal average coil could not pick up.
Another interesting reason that metal detectorists invest in an additional search coil is that they can help them hunt either more of the ground or hunt for a longer period. To hunt more of the ground, metal detecting enthusiasts go for a large DD search coil, this kind of coil can cover a lot of ground with each swing. In big wide open spaces a large search coil is ideal because you can swing widely without hitting any targets.
Metal detectorists also invest in a second search coil if they are looking for a lightweight option. Some DD coils can be very lightweight.
A final reason that pushes detectorists to invest in an additional coil is area-specific problems. The problems can range from high soil mineralization to high trash areas. Finding the proper search coil for issues like these can help you be more efficient and will keep your hunts going smoothly. It is very hard to metal detect in areas with high trash volume if you are using a large search coil, and that is where the smaller coils come into play. Area-specific issues are always important to consider if you are deciding whether or not you should purchase a new coil.
One of the biggest advantages for large search coils is their ability to cover a lot of ground with each swing. You will cover far more ground at the end of the day with a large DD coil compared to a smaller coil. This benefit helps with wide-open areas where there is not a ton of trash.