I think that implementing algorithm instead of using library sometimes can give a better understanding and control of the model. When I am coding some program for scientific computations, it's important for me to understand what I am doing. And implementing the important algorithms helps to get a better knowledge of the problem an achieve a better control on it. Also, sometimes it's not a trival task to select a library that is needed for getting a solution, so it's better to search for already implemented algorithms when you are sure what are you trying to achieve and why do you want it.

If the algorithms are complex, then coding them by hand gives the better opportunity to improve performace/quality of solution using the task-specific features. And sometimes it is necessary to change algorithm a bit, and this is easier if you know the code that you wrote and you can edit it in the way you want.

I think that implementing an algorithm instead of using a library can sometimes give a better understanding and control of the model. When I am coding some program for scientific computations, it's important for me to understand what I am doing. Implementing the important algorithms helps me to get a better knowledge of the problem and achieve better control of it.

On the other hand, sometimes it's not a trivial task to select a library that is needed for getting a solution, so it's better to search for already-implemented algorithms when you are sure what are you trying to achieve and why do you want it.

If the algorithms are complex, then coding them by hand gives you the opportunity to improve performance/quality of solution using task-specific features. And sometimes it is necessary to change the algorithm a bit, which is easier if you know the code that you wrote and you can edit it in the way you want.