https://www.cnet.com/news/galaxy-s10-has-ultrasonic-fingerprint-scanner-heres-why-you-should-care-explainer/

The debate between ultrasonic and optical fingerprint scanners comes at a time when biometric security is on the ascent. In-screen fingerprint readers are a hot trend in phone design because they don't take up any room on the phone face, and require less groping around than a sensor embedded on the phone's power button or back casing. That design dovetails nicely with the move toward an all-screen face with barely visible bezels. 

"Security and biometrics have been integrating into mobile platforms at a rapid pace," Alex Katouzian, Qualcomm senior vice president of mobile technology, said in December at Qulalcomm's yearly tech summit in Hawaii. "This is the future of fingerprint technology."