Technically speaking, design is defined as a plan or a drawing produced to show the look and function or workings of anything before it is made.
In context of internet, design is related to visuals. This includes:
It is an universal truth that what looks good -sells good. Hence you need attractive design which makes your online assets visually appealing to online users.
A well designed website is one of the biggest contributors to online success of your business. Besides, it creates an impression and enhanced brand equity that is expected from website of commercial establishments.
A beautifully crafted website must have these factors:
Moreover, if you plan to generate sales online then design is your best friend. A well designed landing page will give you more sales in weeks (or even days) as compared to traditional method which takes months.
Design is important even if you are sending out an email newsletter. If designed poorly, all your effort will go waste. An email newsletter when designed beautifully will bring in more users to your web assets.
Even the content that you post on your web properties have to be well designed. A well crafted infographic can drive more traffic (and even sales) as compared to a blunt blog post which has only text and links.
If you are using blogging as marketing channel then you should be more serious about its design. If done correctly, it will attract only those who are keen in buying your products/services.
The line "If you cannot measure it, you cannot improve it" perfectly describes Analytics and Monitoring. Your business will suffer if you cannot measure the impact of your effort.