Title tags, Meta descriptions, Meta keyword tags and basic HTML standards are all foundations of your websites SEO (Search Engine Optimization). They do not appear to have the same direct effect on your search engine ranking as backlinks do, but that is because you do not know how they work. Backlinks play into how they work too, but in a different way.
The foundation of your SEO
Title tags, Meta descriptions and Meta keyword tags are all foundation elements of your website SEO. They are what the rest of your search engine optimization bounces off. They are where the search engines start in understanding your website, and are referred to again as your SEO grows and evolves.
Google is trying to understand your website
Google cannot read your website and understand it. Google and the other search engines have to take a look at your website and then try to guess what it is all about. The more accurately they can guess, then the higher up the search engine results pages your website will rise.
Your foundation SEO is all on-page SEO, and that is what the search engines look at first. They look at what you have written on page and what you have done for your on-page SEO, and then match the rest of your SEO with that. If the rest of your search engine optimization is not singing from the same song sheet, then Google has a hard time correctly guessing what your website is all about and so ranks you lower on the search engine results pages.
How useful is your website?
Before you understand the importance of anything when it comes to SEO, you need to understand that Google want websites to be usable. That is the key motivator behind their entire system because the moment it stops being useful then that is when they lose all of their users and user money.
When the search engine tries to understand your website, it assumes that your website has a use. Your title tags, Meta descriptions and Meta keyword tags are all there to help the search engine establish the use of your website. If your title tags, Meta descriptions and Meta keyword tags do not clearly indicate the use of your website, then do not expect to rank very highly on the search engine results pages.
How do backlinks help?
A backlink is what reinforces what Google already thinks of your site. When it first indexes your website it has very little idea about your backlinks. Over time it collects up backlinks for your website, and it uses those to confirm or deny what it already thinks about your website.
If a lot of your backlinks are from reliable sources and reinforce what it thinks about your website, then your ranking goes up. If your backlinks are sending a mixed message, or are sending a message that is contrary to what it thinks your website is all about, then your website search engine ranking goes down.
How does a backlink do this?
There are a number of factors that Google takes into account. The most important is the anchor text that is attached to the backlink, then the website that the backlink comes from and the keywords that the website is ranked for. There is also notice taken of the reputation of the website too, as places such as link farms will have a negative effect on your backlinks as they have a bad reputation.
The search engine will also take note of the text that is in the vicinity of the backlink, and the page that the backlink comes from. There are also other factors such as if your backlink is in H4/H5/H6 tags, if it comes from a directory, the other links on the page, and if you already have a lot of links coming from that website. All of this is considered with your backlinks, and all of it is used to confirm what Google thinks of you.
Why does it make no difference if I change my on-page SEO?
It does, it makes a massive difference, but it takes a while for it to take effect. Google is a slow learner, so if it already thinks your website is about one thing and has collected years of data that your website is about one thing, and then it is going to be a while before it decides you are something else.
Even if your on-page SEO is set up to rank for new purposes, Google will take a long time to change its mind. That is unless you delete all of your old URLS and start from new, but that is going to cause a mess that will see you ranked at the bottom of Google for all queries (whether you are ranking for them or not).