What do Python's pandas/matplotlib/seaborn bring to the table that Tableau does not?
Don't worry - your hard-earned Python skills are still important ;)

Tableau is not a replacement - it is essentially a means of sharing your insights/findings. It is a wrapper around your normal toolkit (Pandas, Scikit-Learn, Keras, etc.). It can do some basic analysis (just using basic models from sklearn), but the powerful thing is it can deploy your models to allow people to run inference on stored data/new data, and then play around with it in an interactive dashboard.

Watch this video for a good overview of everything it can do, and how it connects to Python (and R/MatLab). There is just a bit of boiler plate code around your normal Python code.

Tableau also offer TabPy to set up a server, allowing nice deployments of your work, but in the end you need their desktop application to view the results (i.e. your customers need it to look at the results). This is not free: https://www.tableau.com/pricing/individual

In summary, I'd say Tableau is more of a business intelligence tool, allowing e.g. your non-data-scientist boss or other stakeholders to interactively explore the data and the results of your modelling. Similar to Microsoft's PowerBI.