It is a known fact that unfit athletes (demands will vary from sport to sport, position to position, amount of minutes played, etc) are more prone to making both physical and mental errors. This will negatively impact their performance in their sport, and can make them more susceptible to injury.
Guess what, it is no different when you are in the gym and are strength training. Without getting technical, if your aerobic and/or anaerobic fitness levels are poor, you will be more prone to making physical and mental errors when you are strength training, which will impede your performance. You will also be very hard-pressed to perform anything other than extremely low rep ranges, as you will get winded very quickly. Poor conditioning can prevent you from getting stronger (unless you do extremely low reps), adding muscle, losing fat (if this is your goal), and it can make you more susceptible to injury.
I've worked with some clients who were extremely strong, but were so shockingly deconditioned that they always had to end their sets prematurely (after just a few reps), not because their muscles were overtaxed or because their form had deteriorated, but because their conditioning was extremely poor and they were totally winded. This of course made performing mid to high rep ranges (6-15ish reps) challenging, and made performing supersets or small circuits completely impossible.
Once these clients added in some conditioning (a mix of HIIT, strength based conditioning circuits, steady state (often hiking, biking, snowshoeing, walking), and in some instances SMIT, their overall strength (within a 6-15 or so rep range) dramatically improved, as did their ability to add muscle, and even lose fat (if this was their goal), as their conditioning was no longer the limiting factor.
Conditioning isn't just for athletes, it is for everybody. It will positively impact your performance in your given sport (if applicable), your performance in the gym, and will help you thrive in your daily life. Figure out what you enjoy doing and what works for you, and make it a consistent part of a well rounded training program.