Transactional emails must be delivered almost instantly. Their delivery rates depend on multiple factors, with the most important one being a solid sender reputation.

Sender reputation is affected by good email-sending practices—like minimal bounce and spam rates, clean email content, proper sender authentication and more. Having a separate domain and IP for your transactional emails ensures a dedicated focus required for these emails, helping you achieve a good sender reputation and better inbox placements. 

Further, knowing exactly how domain reputation and the different types of IPs work will also help you improve your transactional emails' deliverability.