Why do we have different types of agreements like EULA (End User License Agreement) and Terms of Service and sometimes other licenses in a legal tab on a website? If we use these terms interchangeably, is there a legal problem?
Why do we have different types of agreements like EULA (End User License Agreement) and Terms of Service and sometimes other licenses in a legal tab on a website?
The names for these agreements describe the subject matter that they cover. They don't have any magical power. You could call an end-user license agreement something else that also describes its purpose.
By convention, however, everyone more or less uses the same names, and that helps people out because they can more quickly and more easily understand what a given document is for.
You can't use the names interchangeably, however, because that would be misleading. The EULA has a different purpose from the ToS. It has different effects. It governs different activities, makes different promises, and describes different consequences for violation.
Similarly, you can't use the names "birth certificate" and "marriage certificate" interchangeably, nor can you use the terms "rental contract" and "contract of sale" interchangeably.
What if we use these terms interchangably is there a legal problem?
There is no legal problem. Those labels can be used interchangeably. The key point is that the conditions ought to be clear enough so that a reasonable person can ascertain them just from reading the document(s).
Consolidating all the terms and conditions in one single document is better than signing multiple agreements. That renders the contract self-contained and tends to preempt overlaps or conflicting terms.
Parties to a contract may choose to have separate but related agreements, or to merge all their agreements into a single larger contract. (Licenses and user agreements and such are contracts, in general.) Having separate, more focused documents can help keep things clearer and more organized, if they are well-crafted. Some of the documents might only apply in specific circumstances. But it is entirely the option of the site operator how to organize such documents. Usually all such documents could be merged into a single "legal terms" document, with no change of legal effect.