Make purchase cost default to null

I don’t know why I need to do this?

database/migrations/2016_08_30_084634_make_purchase_cost_nullable.php

@@ -0,0 +1,29 @@
+<?php
+
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Database\Migrations\Migration;
+
+class MakePurchaseCostNullable extends Migration
+{
+    /**
+     * Run the migrations.
+     *
+     * @return void
+     */
+    public function up()
+    {
+        Schema::table('assets', function ($table) {
+            $table->decimal('purchase_cost',8,2)->nullable()->default(null)->change();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     *
+     * @return void
+     */
+    public function down()
+    {
+        //
+    }
+}