The groupset is the heart of any bike. Yes you may have an incredible frameset with the best road bike wheels, but without a chainset, gears and brakes you've just got an expensive garage ornament. The best road bike groupsets are super smooth and make a the bike move, and importantly stop too.
What's more, depending on the bike you're building, upgrading, or buying new, you have the option of electronic or cable shifting, whilst the industry pushes disc brakes. Rim brakes fans encompass the World as they offer lighter weight options and arguably better brakes that are less prone to locking up mid corner.
Trickle down engineering, from groupsets of yesteryear now have tech from the very top end of the spectrum, making them an enticing option if you're speccing a race bike on a budget.