Investment banks use internships as extended interviews. Your technical knowledge matters, but the broader question is whether the team would trust you as a full-time analyst. That means giving you work under time pressure, including you in sensitive client situations, and relying on you during long or unpredictable days.
Evaluation processes vary by bank, office, and group. Some firms use formal mid-internship and final reviews, while others rely more heavily on informal feedback from the analysts, associates, vice presidents, and senior bankers who worked with you. Staffing levels, business needs, and firm performance can also affect hiring decisions. You cannot control every factor, but you can make the performance case for hiring you as clear as possible.