Why get certified:

Federal, state ,local government and many private corporations give a minimum amount of business (through set asides, preferences, goals) to certain types of  businesses. Being in a certain category gives your business an advantage when bidding on many government and private sector contracts-either directly or through prime contractors for subcontracting opportunities.

Types of certifications

Government

Certifications which can help you get government contracts (as prime contractor) or help you subcontract with prime contractors:

Federal (general info) www.sba.gov/contracting

Small business   www.sam.gov/portal
Small disadvantaged business: www.sba.gov/contracting (go to small disadvantaged business)
Veteran owned business: www.va.gov/osdbu/verification
Service disabled veteran owned business   www.sba.gov/contracting- go to sdvob
8A certification    www.sba.gov/contracting (go to 8A)
HUD Section 3 certification   https://portalapps.hud.gov/Sec3BusReg
HUB Zone Certification  www.sba.gov/contracting   (go to hubzone)

NYS

Woman owned: www.ny.gov/services/business
Minority owned: www.ny.gov/services/business
Service disabled veteran owned: www.ogs.ny.gov/Core/SDVOBA.asp

Nassau County  www.nassaucountyny.gov/1739/vendorcertification

Minority owned
Woman owned

Also, see other government entities, agencies such as

NYC: www.nyc.gov/getcertified
MTA: web.mta.info/mta/procurement/d-m-wbe.htm
PANYNJ: www.panynj.gov/business-opportunities/sd-become-certified.html 

Private:

Private companies such as Fortune 1000, Fortune 500 companies look to do business with certain types of companies-minority/woman/veteran owned for example. Some of these private sector corporations do in house certifications. Many work with intermediaries who do the certifications:

National Minority Supplier Development Council (certifies minority owned companies): www.nmsdc.org

NYNJ Minority Supplier Development Council (certifies minority owned companies): www.nynjmsdc.org

NWBOC: (certifies women owned businesses for private corporations & certifies veteran owned businesses for  US Government): www.nwboc.org

WBENC (certifies women owned businesses): www.wbenc.org