Business License: 

One reason the government wants you to obtain a business license is a general small business tax ( $10 to $50 or more annual, usually, flat tax)  they impose on you. Another is the regulations the subject you too.  For example, no commercial signs showing from your residential home business, etc.   

All businesses are legally required to get one because either the state, county or city will not let you legally start before you register with them, follow their rules and pay them a small BUSINESS tax ( as opposed to gross INCOME tax or NET INCOME tax, SALES tax, EMPLOYMENT tax, or FEDERAL income tax).  

Note that a corporation, a LLC, or a DBA, Seller's Permit, State or Federal Tax ID Number, is NOT a business license -- this is a required business registration separate from the others.

Seller's Permit..  Legally required to buy or sell wholesale and sell retail.

Federal Tax ID

To use as a business tax ID, to Report Fed Employment Taxes, to open a business bank account, to identify a partnership, to work for companies as an independent contractor, etc.

DBA Trade Name Registration.

Legally required to register your business name, open a business bank account, advertise in the company name, protect it from others etc.

State Employer Tax ID

To report and pay employment state taxes.

LLC agreement

Free if you purchase the LLC filing.  Required to open a bank account.   In addition to the LLC registration certificate, you will need this to open a business bank account because it is the only document that state who owns what in the LLC.

Corporation

Incorporation is not legally required but recommended to save on taxes and protect your personal assets.

Business Name Search

Required if you want to make sure no one locally or nationally has or uses your company name.