Laravel Container 中 为什么要用 $concrete === $abstract 来判断是 build 还是 make？

<?php

class Container
{
    protected $bindings = [];

    public function bind($abstract, $concrete = null, $shared = false)
    {
        if (!$concrete instanceof Closure) {
            $concrete = $this->getClosure($abstract, $concrete);
        }
        $this->bindings[$abstract] = [
            'concrete' => $concrete,
            'shared' => $shared,
        ];
    }

    protected function getClosure($abstract, $concrete)
    {
        return function ($c) use ($abstract, $concrete) {
            $method = ($abstract == $concrete) ? 'build' : 'make';
            return $c->$method($concrete);
        };
    }

    public function make($abstract)
    {
        $concrete = $this->getConcrete($abstract);
        if ($this->isBuildable($concrete, $abstract)) {
            $obj = $this->build($concrete);
        } else {
            $obj = $this->make($concrete);
        }
        return $obj;
    }

    protected function isBuildable($concrete, $abstract)
    {
        return $concrete === $abstract || $concrete instanceof Closure;
    }

    protected function getConcrete($abstract)
    {
        if (!isset($this->bindings[$abstract])) {
            return $abstract;
        }
        return $this->bindings[$abstract]['concrete'];
    }

    public function build($concrete)
    {
        if ($concrete instanceof Closure) {
            return $concrete($this);
        }
        $reflector = new ReflectionClass($concrete);
        if (!$reflector->isInstantiable()) {
            echo $message = "Target [$concrete] is not instantiable";
        }
        $constructor = $reflector->getConstructor();
        if (is_null($constructor)) {
            return new $concrete;
        }
        $dependencies = $constructor->getParameters();
        $instances = $this->getDependencies($dependencies);
        return $reflector->newInstanceArgs($instances);
    }

    protected function getDependencies($parameters)
    {
        $dependencies = [];
        foreach ($parameters as $parameter) {
            $dependency = $parameter->getClass();
            if (is_null($dependency)) {
                $dependencies[] = null;
            } else {
                $dependencies[] = $this->resolveClass($parameter);
            }
        }
        return (array)$dependencies;
    }

    protected function resolveClass(ReflectionParameter $parameter)
    {
        return $this->make($parameter->getClass()->name);
    }
}

interface Visit
{
    public function go();
}

class Leg implements Visit
{
    public function go()
    {
        echo 'walk to Tibet!';
    }
}

class Car implements Visit
{
    public function go()
    {
        echo 'drive to Tibet!';
    }
}

class Train implements Visit
{
    public function go()
    {
        echo 'go to Tibet by Train!';
    }
}

class Traveller
{
    protected $trafficTool;

    public function __construct(Visit $trafficTool)
    {
        $this->trafficTool = $trafficTool;
    }

    public function visitTibet()
    {
        $this->trafficTool->go();
    }
}

$app = new Container();

$app->bind('Visit', 'Train');
$app->bind('traveller', "Traveller");
$tra = $app->make('traveller');
$tra->visitTibet();

在getClosure 和 isBuildable 中 为什么要用 $concrete === $abstract 来判断是build还是make。不是理解的很透彻。 像这样的容器是怎么一步步设计构建出来的