IN a website you may need to insert many tags or codes on various pages for different purposes. This will make youe website heavy and in result there will be high load time
In order to rectify this issue,we use tag manager where we can insert multiple tags and we just need to insert one google tag manager tag to our website. 

Thank you for the information! keep sharing!

hello all,
Can anybody tell me what is Google Tag Manager?

Hello all,
Can anybody tell me what is PBN in SEO?

thanks for sharing your  ideas

Site map is the one of most important tactic in seo, in that many of the people don’t know use of site map and the importance. so I will tell simple answer for your understanding purpose. search engines they don’t know about our site and how many page in the site and content etc. for that you are telling to search engine through site map you are going give the information for search engines than search engines can easily recognize   

Online, it is only a matter of the customer searching for the product in a search engine.

All the above three source WordPress, OpenCart and Abantecart  are the best but you did to check the in depth working analysis and functionalities of each one.

As gordontaylor described is the best way to avoid stuffing of keywords in the web page. Use keyword density tools to limit the keywords and recognize the perfect keywords.