All the defenses in the world are completely useless if they are pointed in the wrong direction.

Are you only protecting yourself from the outside in? Are your only defenses at your parameters as the main line of cyberattack defense? Keep in mind, there could be many attackers and they are extremely patient. They can wait for months to gain access to your systems. Hackers will sit quietly within your infrastructure, watching, gathering information and waiting until the right time to exploit the weaknesses they have found.

In some cases, hackers write custom software just to attack you with the information that has been gathered from your internal network.

You may be thinking, "Not my company! We have huge security expenditures to keep the “bad guys” out!"  If that is your belief, I must point something out: Most companies, large, medium and small, across most verticals, have their defense posture from the outside in.
That leaves you wide open to a phishing attack. Do you think you are immune from this? RSA is one of the most advanced and well-known security companies. They have secured millions of users.  Some of the largest companies on earth have used their “SecureID” product. They were attacked and compromised in 2011 with a phishing attack that lead to the compromise of the RSA “SecureID” database. This caused a snowball effect with their customers.

Keep in mind that RSA was not necessarily the end-goal target of this attack.

It might have been one of its many customers that used the “SecureID” product. This is just one example of the millions of attacks that impact companies worldwide.
Some of these attacks might just target a company because who their clients are. For example, a bank might get attacked because their cybersecurity is porous. However, they are part of a larger network, like a credit card processing company. This then exposes the entire connected partner network from the inside.
Exposure might be something as basic as not patching a known exploit. This happened in Equifax's attack that exposed the Person Identifiable Information (PII) data of millions of people.

Your company maybe heavily secured, the attacker will look for a weak spot in the chain to exploit.

That type of attack might just expose your company if you are isolated from the rest of the world. However, the idea of “air gapping” is rare these days as more companies rely on outside sources to do their business and share data with outside sources. If your partner company, the one you rely on, has a weak cybersecurity posture, you might fall victim if they are successfully attacked.
Sometimes the attacks are politically driven. Others just want to see if the attacker can exploit known security bugs, while some are for financial gain.

An attacker will do what it takes to gain access.

They will look for any weakness to compromise. This might be a head-on attack against your outer defenses. Or, if that fails, phishing attacks could compromise you with an unsuspecting employee. They might click on something that lets the attacker inside or could even be USB keys in your companies parking lots, that are inserted into an employee's PC. These are just highlighting some ways that open the door to attack, but this is by far not a complete list of ways an attacker might use.
Whatever the motivation of the hacker is if they try to attack you, are you ready? Do have all your bases covered? Or will you just be the next victim?
Ensure you are not only securing your infrastructure, but also educate your employees about phishing and other types of cyberattacks.