You need an accurate cap table because you will be constantly making decisions that impact equity within your company.
By knowing how ownership is divided within your company, you can answer questions like:
Hiring - how much equity should I give early employees? How many shares should I set aside for an employee option pool?
Fundraising - how much dilution will I take when I fundraise? What is the difference in dilution between fundraising options?
Do I need a cap table provider if it's just me and another founder?
No. Most companies upgrade to a cap table tool when they hire and when they fundraise. If it's just you and your cofounder, a spreadsheet works fine.