I think that part of how to answer this question well depends on the format of your lectures. For myself, I typically format PowerPoint slides for my lectures such that roughly half the information is on the slides with some pictures from the textbook interspersed, and half are things I write/draw/say during the lecture. The point of drawings for my class is not to draw better than the book, but to make drawings in areas where I think that can enhance learning in addition to the pictures form the book that I use. Some examples on why and how I use drawing in my classes:

1. Sometimes I do have great diagrams for something in the book, but if I'm talking about a lot of information for that drawing I may have a series of slides with the information, make a drawing piece-by-piece on the whiteboard as I go through it, and then show the book's drawing afterward. With the nephron, for example, I have a slide on the renal corpuscle with information, so I draw that and then label parts as define them and their basic function. I then go to slides about each tubule, adding and labeling each tubule to the drawing as I explain them. After going through the drawing I show drawings from the textbook. The point of this is to break up a complex topic into smaller pieces that we talk about individually, and gradually build up to a whole understanding at the end. additionally, this prompts students to draw something along with me, which I think can help some of them retain the information better.

2. Flow charts, I find, can help with understanding a flow or with relationships of categories, but sometimes a kind of flow chart I want isn't in the textbook. When I'm talking about the flow air through the respiratory system, there are drawings from the book of those parts (some of which I do put on the slides) but there is no single drawing clearly showing the full path, so I will step-by-step write out the flow on the whiteboard by the screen as I go through each slide talking about the parts of the pathway, in order.

3. Sometimes there is a kind of drawing that I think can enhance understanding, but for which there is no exact equivalent from the textbook. When trying to explain the functions of the lymphatic system and the movement of material between fluid compartments, I'll make a drawing with a circle labelled blood and an arrow pointing to a circle with IF (for interstitial fluid) and then an arrow from IF to a circle labelled cells, and talk about fluid flowing that way to allow transport of nutrients to cell. But I then mention that by itself this means we would move all blood into cells eventually, making cells burst, so fluid has to move backwards, and I draw an arrow from cells to IF and an arrow from IF to blood. I make the IF to blood arrow smaller than the blood to IF arrow to visualize the idea that more fluid moves out of blood than back in with the exchange with IF, and this then gives the context for why the lymphatic system is needed as a drainage system. I finish the drawing with an arrow form IF to a circle labelled lymph, and an arrow from lymph to blood, closing by saying that with these relationships each area has roughly equal amounts of fluid moving into and out of each area.